I've been lurking on the Blitz forums for years, because I have always been interested in BlitzMax with the cross-platform support, OOP, etc. I have yet to purchase it, though.

Blitz has zero self-promoted advertising and is all word of mouth, whereas TGC is all over the place with business connections galore (DarkGDK on M$ website). And the online communities are different in many, many ways.
There are many other differences between the two companies. TGC has $ coming in from various sources like Driving Test software, popular game-makers like the two FPS Creators. As far as I know Blitz just has the languages and the SDK. They were working on a game maker type product but it fizzled out.
Like it's been said already, BMax has many user-made modules wrapping standard 3D engines, and the MiniB3D module (written in Bmax to emulate Blitz3D) is feature complete from what I've seen. The official Max3D module (open source) is still a bit crude still.
If you need to produce cross-platform code then obviously Bmax (or something similar like purebasic) is your choice. From what I've read (from Bmax users) in order to utilize Bmax to the fullest you need to design the project with object oriented features in mind. Along with that, some of the nifty features of Bmax (low level control of data structures, pass by ref, etc.) mean it has the chance to be harder to learn for beginner programmers.
At least that is my opinion.
But I don't think anybody can say that DBPro is that hard to start out with and I don't think anything out there is faster for prototyping ideas and rapid development of a 3D game. I mean, if I can program in it, ANYBODY can.
Great forum, great community, great newsletter, easy language... what else you need?
I'm not a real programmer but I play one with DBPro!