Here's my $.02:
I bought a Spirit 23 track body about 6 years ago. It was my first body. I went and picked it up at Spirit and was happy with it for the price. It was a bit tweaked and needed a bit of body work...I ended up selling it as I decided that I wanted something a bit bigger--so I bought a poliform 27 stretched body. Man what a difference! Like others have said, you get what you pay for. I still had plenty of fiberglass work and body work, but I realized that with the Spirit body I would have had much more time in it. With that being said, I still think Spirit bodies a a great value. I think they are definitely better than the speedway bodies, and they have great service. If you are the "I'd rather save $200 and spend another 20 hours" type guy, go with Spirit. If you have a few more bucks and less time (or want less frustration) go with one of the others that make a premium body.
As for MAS, I've heard all the bad things about their bodies...I've never seen on in person, but I did buy some 4 link bushings from them and was very happy. I'll be in the Twin Cities later this fall and plan on dropping by their store. We're planning a "beer-budget" T and want to check out their axles and bodies. I'll give a full report!
Oh, one last thing: from my experience, if you want a door(s) for your T, pay to have the body mfgr hang them...I couldn't have been more frustrated and spent more time dealing with that than anything else!