In most ways, this an improvement over the previous "new costumes" for Superman, but then again, the previous "new costumes" were severely flawed, so is that saying much?
Speaking for myself, I DEFINITELY would pick the classic trunks costume. Now, let me be clear: I get it. I get why the trunks look can be considered to be silly-looking. Sometimes I look at the classic costume and, frankly, I get it.
But that completely valid point notwithstanding, I think Superman's classic costume is an excellent design overall, and yes, removing something as major as the red trunks does mess it up considerably. Even if the results aren't bad, as in this case, it's still a step down. And to me, taking something amazing and taking it down a notch is kinda worse than just making something outright bad to begin with.
It's not that the color balance of this new costume is horrible. It's that the color balance on the old costume was great. I think the "breaking up" of the costume is indeed hugely important and just looks really, really good. Not to sound silly, but because of shirts and pants existing in modern society, people are used to there being a very clear and distinct division between the top half and lower half (legs) of a person's body. The trunks provide that; making the lower half of Superman's body completely covered in the same color as the upper half doesn't. (Obviously, the situation isn't the same as a person wearing a shirt and pants of the same color, since the shirt and pants are still obviously separate unless they're very tight-fitting. Superman's costume is very tight-fitting, of course, so he needs something else to provide that division.)
Superman's body being entirely covered in blue just doesn't...look good to me. He looks like he's wearing a continuous full-body suit that's a single color, and I just don't find that appealing. In addition to looking utterly abnormal in a society where people just...don't wear full-body suits that are a single color (and if they do, they look really weird), and it's also bland. Superman's costume should be really, really bold, not bland in any way. The crucial presence of all that red at the half-way point on his body really provides that.
I'm not going to argue that you shouldn't find "wearing your underwear on the outside" to look silly. But I will argue that it's so, so, SO worth it. The way the classic Superman costume can be so dynamic and so simple at the same time is just what makes it so classic and legendary, and how it's endured so well over the years. The fact that all the attempts to amend Superman's costume are always short-lived says a lot about their timelessness...namely, that they have none, lol. The classic costume has endured for approximately 70 years with very little modification, while all the new attempts to make a costume for Superman can only seem to stick around for a few years. Isn't it time to give up and accept the "underpants"?
Again, I'm not here to convince you that the trunks don't look "silly". I just don't see why this rather superficial "silliness" should take precedence of more important design considerations, such as the overall boldness of the look, color balance, and providing that sense of the costume being "broken up" at what is both the halfway point on the human (or Kryptonian) body and the beginning of the legs. Basically, accept a small and arguable flaw to receive a host of clear, valuable, and less arguable and subjective (notice I say LESS, not "not at all") advantages. The trunks aren't perfect, but if you accept them, you'll hopefully find, as I did, the classic costume to be the bold, beautifully simple, iconic design it is.
(For the record, I also really prefer Batman having trunks too. I love the clear division and design balance they bring.)
Then, there's the consideration of the classic design being traditional and iconic. To me, this is actually quite important. Superman has a lot of value out-of-universe as one of America's most iconic classic characters. His past extends to before World War II even began. To me, this is a great asset, not a liability. I love the fact that I can connect with people wildly different ages from me because of Superman. So to me, embracing the tradition of Superman's character, including his design, is an inherently good thing. I'm not saying we should embrace tradition to the point of never trying anything new, but just embracing his traditional character design doesn't seem to be going to far, especially considering how consistent it's generally been over the years.
Even out of universe, people look to Superman as a symbol of hope and of unchanging ideals of good triumphing over evil. To me, when we keep Superman looking basically the same as he always did, the same as when George Reeves was on TV or Christopher Reeve was on the silver screen, we are symbolically embracing the unchanging nature of that reality. It's not that we HAVE to, it's just a small way of saying that Superman is the same, and we want to maintain what Superman is and what he represents. I mean, why not do that? Of course, we can maintain Superman's ideals and who he is even if we change his costume - I'm certainly not saying otherwise! But it's natural to associate someone's appearance with who they are. I don't want Superman to be different, so why should he look different?* Now, look, if there was actually something horribly, glaringly wrong with Superman's costume, I would get it. But there isn't, so...why even risk marring Superman's symbolic value in the slightest if we don't have to? What's the point?
Maybe it's just me, but I feel that even the superficial things we do with a character can say something about how we're interested in using them. To me, it just doesn't send the best signal to change Superman's look, even if it's "harmless" in and of itself. To me, it says, "We're going to arbitrarily change things which are good as is, not to truly improve them, but just to make them more modern and/or just for the sake of change itself." To me, that's a BAD signal! Sort of a "being faithful with little, being faithful with much" kind of thing. Combine that with the design considerations mentioned previously, and it just seems like changing Superman's design is just totally not worth it. I mean...all this just to get rid of the "underpants"? Really?
I also have a real problem with the idea of Superman wanting to "look less silly" in-universe. He doesn't NEED to not look silly. He's Superman. People take him seriously because it would be stupid not to, lol. Also, doesn't Superman care about what his own costume symbolizes? Shouldn't those out-of-universe considerations I cited apply in-universe too? Wouldn't Superman care more about being an unchanging symbol than keeping up with fashion trends?
OK, so us trunks fans may be a bit obsessive. Or at least I am. lol! But we have real concerns that I don't think should be dismissed, overlooked, or considered "not that big a deal". It's clear to me that the obsessive need to get rid of Superman's red trunks has brought on apparently endless redesigns because nothing can quite match the classic, and I honestly don't know when this will ever end unless we just accept the classic design. As I've said throughout this post, it just isn't worth it!
*(Now, of course Superman has changed IMMENSELY over the years. The very parts that haven't changed, first and foremost him being a symbol of good, are the very parts that are most important to who he is. Superman's costume has also not changed much over the years, so to me it's a symbol of the parts of Superman that don't change - which, as I just said, are the most important parts about him.)
tl;dr Trunks reign supreme.