I have had the pleasure of driving 2 of the Rudy V-8 cars and they are amazing. Jim's car is running 11.2's and with Rudy in town turning wrenches sure to make improvements. My car is an absolute blast to drive but my dream is to some day get a Rudy transplant. Each time I drive one of thoes cars it takes weeks to get it out of my head.
So if you have the resorces the V-8 would be my vote. On the other hand an SRT 6 to start modding will keep you grinning for quite a while. You really can't go wrong either way.