as a guitar player who has played allot of his work in creed and alterbridge i never thought the style and sound of his guitar playing was that different to a point where he could'nt addapt back to creeds sound ..i mean his playing is allot more intricate and might have more parts, but from what i heard from full circle and those rehearsal recording of devil inside, sounds like he's just about got creeds sound down pretty good still ..for example the song Rain, i think it could fit on Weathered pretty nicely ..maybe next to lullaby. basically alterbridge songs just had allot more palm muting and solos, which are not foreign to creeds sound anyway ..so i think he'll be fine.
As far as songwriting, all of Marks work is stellar over anything else these guys put out. Marks writing with his solo work shows what he is to Creed if you ask me. I also think it's why so many people like the first AB album so much and not the other ones, it was Marks writing.
probably not as good ..i'll admit myles had the pipes, charm and stage presence to push the band in the direction they needed to go ..if mark was singing they'd probably end up playin small shows in bars and clubs like Tremonti project. as talented as mark is, i don't think the band would be where it is today if mark was fronting the band.