In either case, I say add both features if you can. Sometimes you want to explore, sometimes you just need to get from A to B, but at all times you should feel FAST.
And while I'm coming up with a pipedream wishlist, I hope someone makes an engine with action sequence replays. You simulate superspeed by slowing the game down, but then if you want to see how that played out in real speed, you go into replay mode and show us what it looks like when you use a time elapse simulator, with blurring and everything.
And slo-mo action has a physics compensator. That is, if you "slowed" your environment to running at 1/10th of actual speed, that means you pack about 100x or however much impact (sorry, my physics is so rusty...).