I've been to both and personally I think that the food is the same. However, the decor of the Tribeca location is terribly kitschy, kind of like a caricature of Brazil. The mid-town location looks like a rodizio would in Brazil. Porcao is also good, but every time I've been there it's been dead, not the ideal location for a bachelor party. Go with mid-town...
I can't really speak to the Tribeca location. I have been the midtown location three times, though. The first two times were really fun -- the service and the food were top notch. My most recent trip there, maybe a couple of months ago, wasn't as good. I dont know if that means the midtown one is slipping. I'd be interested to hear what others think too. Is the Midtown location slipping? Is Tribeca better?
Or, even if there is a better rodizio in the city? I've been to Porcao, and it was fine, but not as good as Plataforma.