Why? Mining with BMM is a great way to make money as a team. It's about as profiatable as trading. Trading in Bowex or Gateway convoys is better than slugging it out on a random trade run.
Yes, join Gateway! I've escorted them a few times, they're a great group to play with. If my activity ever picks up, I might make a Gateway escort fighter.
You'll get good RP, plenty of security (nothing says "don't pirate me" like a convoy of Shires!), and plenty of laughs.
I don't mean to say the other factions aren't that good, I just haven't had any experience with them.
I can personally vouch for all three factions. They're all good RPers, and full of great people. Though, I must admit that I am partial to Bowex for some reason....;)
I wouldn't mind seeing a larger, more active Gateway, though. It's a lot of fun when a Gateway and Bowex convoy meet, and I've been missing that bit of RP.