Both very good choices. The newest RSX (05-06) is better looking IMO, and a little less common. It's a little more powerful too, since it got some breathing mods plus ITR cams & throttle body, along with a slight facelift. It also has a more upscale interior than the Si, with leather if that appeals to you. They're pretty much close to identical in the powertrain department so as far as modifications & aftermarket support you're covered either way. I prefer the RSX, but they're both great cars.