That's another difference that I noticed, though I spent more time fighting revenants than arch-viles and thus I thought about it more often. Revenants don't always fire a seeker missile (which doesn't always hit), and Arch-Viles need to 'charge' their spells. A chief part of FPS strategy against them is letting them attack you and ducking into cover to avoid damage. Oh, and they can't ress while moving and not practically in melee range... all of this reminds me of the issue I had with the mancubus firing a burst of three fireballs instead of three bursts of two, which could probably be implemented as a sort of chain-fire attack that can't be stopped until turn three and is always fired in the player's general direction regardless of LOS.
Why would different attack and movement speeds be an issue? It's already quite clearly implemented for the marine.
Edit: Oh yes, I'm also of the opinion that revenants would be a bit scarier if they came charging at you like barons, throwing their terrifyingly dangerous contribution to whatever's on your plate at a given moment.
