man!
Differences between EX and Si are pretty substantial... similar body styles, but completely different motors. EX has a D16 along with the rest of the 6th gen models (except Si). Si has a B16, which should be you choice if you plan on doing any performance mods. If you want fuel economy, stick with a D-series motor.
Feel free to post any questions you have in the appropriate forum section and I'm sure other members will be glad to help you as as best they can!