I actually don't picture movie version for either, even when it's a movie fic. I also have a hard time picturing comic version because their looks do change quite a lot from artist to artist. Tony at any time looks classic to RDJ to sunburnt Italian to nondescript pale skinny guy with black hair and a van dyke. I usually picture the latter.

Steve can go from either MUSCLES to just athletic in the comics. I definitely can't picture Evans in this role. But Marvel Adventures style is usually how it goes. Not too square-faced, not too huge, a nondescript guy with blonde hair and a body like an offensive lineman.