Well both Shia and Paul have both played the cocky upstarts and I've only seen them in those roles.
Granted Shia has been in roles where he gets thrusted into a war aka Transformers (haven't seen Eagle Eye yet), but he's just a kid that get a lucky shot and never matures or actually gains any skill. With Paul Walker, I've never seen him do a revenge role, let alone see him being a tactical genius with something other than CAR CHASES.
As far who would be better for their roles, I would have to do research cause I'm no film student that truly analyzes the actors and everything else. This just seems as bad as having Ben Affleck as the new Batman.