Yeah, I've got both systems too... I switched from RAD to GameStudio a couple of months ago. Some reasons I moved to GS:
RAD does not have deformable models - the only way to have animated human models is to create a hierarchically animated multipart model and convert it to .x format. If you want a single mesh, you have to use frame-by-frame animation which will mean as many .x files as you have frames (and there's no interpolation, so it's jerky).
Polygon collision has just been introduced - but as I understand it, in the 'brain' (script) you have to declare every combination of possible polygon parts that can collide. I might be wrong on that, but that's how the new brain reference reads.
RAD does have 'dynamic' shadows... but you have to declare in the brain every possible plain and angle at which they can occur. ie. Unless you're on flat floor, forget it. Oh, there's no shadow mapping either.
No particle system, no strings.
RAD does have cheap multiplayer, but you have to create every player at game-start - and given the 256 element limit (although there's now a workaround for that), this is very limiting.
Oh, and the creators recently had to threaten to revoke future upgrades just to stop the childish behaviour on the forum.
That said, it's good for racing games - Someone's Wraiths is a great Wipeout style game that was made with RAD. And I just put some animation script into a very small multiplayer Tribes/Quake style game made in RAD by Someone and a 3D artist which was looking pretty damn good last time I tested it...
Pros and cons, but for human-based games I'd choose 3dgs everytime.
Whew, that was along post. Not at all obvious I'm putting off uni work, then...
-Keith