I have no previous experience with Diggle, but I'm actually really familiar with Christos Gage's work, and I totally understand your apprehension. He's written Buffy and Angel & Faith for the last 5 years, and some it has been transcendentally awesome, but some of it has been rather bland.
That said, as a hardcore Valiant fan, Gage is absolutely killing it on Ninja-K. It's easily the best Valiant series coming out right now, and it's doing a beautiful job at tying itself into the wider Valiant U. It really feels like a book that's happening now, if that makes any sense.
Shadowman, on the other hand, isn't really my cup of tea. It's gotten good reviews on here, ValiantFans, and some podcasts that I listen to, but I'm just not feeling it that much. It seems to be a whole lot of exposition about mystical/voodo-y stuff that I just don't care about.
Full disclaimer, I'm really not into magic superheroes.