Most people posting here are happy with their cars. The Vintage owners will recommend Kirk Duncan; the JPS owners will steer you to John Steele. However, there ARE some differences in the two manufacturers' cars, which may or may not be significant to you.
Rather than toss your "Which is better?" question to the partisan troops at SpeedsterOwners.com, why not decide what you're trying to accomplish with a new Speedster (how you plan to use it, how much HP do you need, how equipped, etc.) and then talk to both Kirk and John and see what they say.
It was my experience that price was not a factor - both Vintage and JPS were within a few bucks of each other for similarly set-up cars. (IM is in its own high-priced universe.)
The details were very important to me and that's really how I made my decision. Both guys make very nice cars.