I'm on TMO and bought an unlocked AT&T curve. I bought it because TMO doesn't have the red one. WiFi would be useless to me. Didn't even think about it having the GPS but I've been playing with it and decided I really like it. (Especially after doing John Clark's free up max memory protocol. Part of that takes out the AT&T vendor file and loads BB Maps. Works like a charm.)
Anyway, buy which ever version you want regardless of your carrier. I have previously had unbranded phones and they were frequently a pain to get all the settings correct for internet, etc. With the blackberry, it's a piece of case since the service books take care of it all.