There are a lot of differences. Besides the engine difference which someone already quoted the SI has:Red interior lights instaed of blue17" rims instead of 16 (EX)350 Watt CD/MP3 stereo w/subwoofer instead of 140 wattSi stitching and black interiorSpolier in backChrome exhaust tipSi floor mats3 Si-only colors (Rallye Red, Fiji Blue Pearl, and Habanero Red Pearl)Aside from that it's an awesome car.