They compare equally. There may be a bit more work on the S3's crank. Maybe a hair more power. Other than that, the engines are basically the same. I have an X12T. The crank has some decent work done to it. Replace the turbo button with a std button and you're good to go.
Cachi, I had the same problem. Don't bother with the S3. Actually, you can order it for $296 off of trinity's site....ROFL. I emailed them and told them where to go. If you go to www.ashfordhobby.com, they have the RB-X12 210's for 179.95. I ordered mine last night. Should be here Wednesday.