What is the advantages/disadvantages of the SMB connector against SMA?
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.