A. UT3 Engine sucks. All static environments. No physics but player physics. It's laggy. The aliasing sucks. And it's a little bit hard to mod for.
B. Cryengine 2 looks good but. Most PCs can't run it, unless it's very laggy. You have to have models for buildings, witch takes away the reason for being a mapper. It takes skill to make brush or mesh buildings. But if you have a modeler to model building there is no reason for a mapper.
C. Source is simple, clean, and has very good physics. Easy to mod for (except for coding

). Lighting is really good in ep2 build. The modeling is a little dumb though. Six different files just to make a model look right.
D. Most games are very limited in modding.
I would choose source in 5-10 years. Just update the engine a little bit. I believe that game quality has pretty much reached it's limits for a LONG time.