Kyle,
The chassis are pretty much the same except the coupe's roll cage cuts back right behind the driver's head. I was told the sedans are a little faster, but we've been beat by both and have beaten both.
If you are going to be racing at an INEX sanctioned points track, you may want to get a sealed 1250 engine because they will tear down the open motors quite often. Just as in karting, there isn't a huge difference between a good engine, and the best engine. But the real difference is in the setup and the driver.
We bought a used 1937 Chevy Coupe with a sealed engine that had been just been rebuilt by GE Chapman. I then bought a setup manual from Kevin Yeatts. (available online) With some minor tweaks, we have been able to adapt the setup and do well. Once you have the car dialed in, you can adjust it with air pressure. The engine will run good for quite a while. We have put over 40 races on ours this year, and it is now down for rebuild. By the way, that's our car in the upper right hand corner of this site.