I think the CRX is faster. All things equal other than wheelbase and body shape, I'm sure the CRX is a better racecar than the Civic. If it's a little more squirrelly than a Civic, so be it. Run softer springs in the rear!
I race with the same Club as Chris, and ALL the GT3 cars are Civics.... and the guys running them claim the wheelbase is an advantage, but if you ask them if they ever TRIED a CRX, the answer is "well, no...."
Personnally, I'd rather lose the extra foot off the back of the car, and have a slightly lower roofline, and less weight, and better looks, etc.