I have a 07 FG2, Its great and pulls pretty damn hard. The 06+ Si are fairly new so there isnt as much mods and programs as the RSX yet, Give it a little bit more time and there will be alot of stuff out. There is a turbo kit called AJP for the Si and it can put down 375+WHP on stock internals.
Si Pros-
Better suspension, Wider and Handles amazingly
Bettered geared LSD Tranny
Looks imo
VSA (Traction Control which you can turn off
)
Interior looks alot sportier and is hi-tec looking
More aggressive sounding
RSX
Leather seats if you like luxery (I perfer my Si cloth anyday)
More parts and tuneing options at the moment.
Cable throttle (Si is DBW (Drive by Wire) so the throttle response isnt as fast but you cant really tell)
Curcuit race where there are alot of turns the Si wins, From a dig or roll its a drivers race. Cant go wrong with either car but I perfer the Si