The principal difference is the major one:
- SMB connectors are Snap-In. You push them together to mate, and pull them apart to unmate.
- SMA connectors are threaded. You have to screw them together, and unscrew them to disconnect.
SMB connectors are also slightly smaller.
SMA connectors are also far more common.
Personally, I would be much more inclined to go with SMA connectors, just because they're so much more easily available.
However, since this is going outside, I would more strongly recommend SMA, as it is much more mechanically reliable.
However, it's crucial to note that the Bulgin connector you link isn't really SMB. It's a SMB-style connector with a bunch more stuff on it. It's also exclusively available from Bulgin. However, realistically, any high-frequency IP67 connector is going to be non-standard (well, aside from N connectors), as there aren't really any standards for sealed high-frequency connectors, so the fact that it's Bulgin-Exclusive isn't really a big deal, as anything else will be similarly single-sourced.
On the whole, If you really need IP67, I can highly recommend the Bulgin Buccaneer connectors. I use several varieties of them in a device I designed, and they are very nicely made. Just be aware that you will need to make your own cables (which involves crimping/terminating coax), if the few varieties Bulgin offers do not suffice (and the longest cable they do make is 5M).
Is there any reason you can't use a N connector on your equipment? The only real downside to N connectors is they're large.