Different people have different opinions about which is the best to go to. I think there have been past threads about that here...you may want to try to look them up?
The benefit of one of the larger offices (like St. Mary), is that they are likely to have two "stations" processing people at the same time. So, if you are second in line at St. Mary, the rangers will get you "onto" the system faster than if you were second in line at, say Two Med, which has only one computer terminal.
On the other hand, more people tend to show up at the larger offices, so you may have to get in line earlier there to get those coveted first or second spots than you would need to if you were at a less popular office. I've gotten to St. Mary at 5:00am and been third in line. And gotten to Two Med at 6:30am and been second.
Since all the offices (except Polebridge) open at the same time, and are all hooked into the same computer system, you are competing with all the "first in line" folks at ALL the offices.
So, if it's important to you to get one of the really popular campsites -- especially if it's less than three days into your itinerary and the weather is looking good -- you will want to be first or second in line at the big offices, or first in line at the smaller ones. If your change is farther out -- say 4 or more days into your itinerary (or if the weather is looking miserable or if you are trying for a less popular site) -- you'll probably do OK even if you are later in the line.
Yeah, it's a bit of a crap shoot. But remember, there are no bad hikes in Glacier.
Just make sure you have looked at the website the night before so you know what is and is not available, and write your itinerary choices and alternates down so that you can work through your options with the ranger as efficiently as possible once they get into the computer system. It will work better for you that way, and you'll get a lot less in the way of bad juju stink-eye from the people waiting in line behind you.