I can't say how the 12 " stall from Edge is but the 9" is amazing. turned my car into a total beast. Went from fairly quick to smoke show on demand. When I installed mine with some Et streets I picked up over 1.5 sec in the 1/4. No other changes.
If the car is going to be totally a street driven car, I'd look into something in the 2600 range, and 3000 if you are looking into a true street/strip car. If you do alot of highway driving, you might want to go lockup and non lockup if it sees some strip action. You'll loose some mpg but well worth it for the improved strip performance.
Give Andre a call at Edge and talk to him. He'll spend as long as it takes to answer any questions you may have and can give you a very good explanation of how these things work.