Well, thought I'd add my two cents worth.
First, my own personal biase is towards a busy forum and Atlas is a busy forum, probably the busiest of all the forums. Because it has so many active members, it is harderd to get that "small town" feel to it. But I like the spontaneity, the "quickness" of the pace.
I agree a fast pace can be difficult to follow, especially if you only visit sporadically. Threads do have a way of falling off the edge of the earth there.
But you also have the most manufactures visiting that forum, and occassionally getting into trouble. The MP president did a bang up job of it (in a negative sense) when an N scale forumer (Steamguy) gave a very critical review of the first version of the Pacific in N scale. I also noticed a few posts by Terry Thompson (of MR) recently, but I think he will find it safer to stay low key there.
The guage has a "small town" feel to it. More longer term folks, good modellers, scratch builders, kit bashers, everything you need. It is a well done forum. But I like the fast pace and diversity of a large city, rather than the slower pace and less diversity of a small town.
I think the crowning jewel of this forum - for me - is the storage and archiving of good ideas; this is invaluable. When ever anyone asks about backdrops for instance, I always provide a link to here.