Personally,I think Gloom is a better game.Unfortunately,the server browser is non existent,and the game is somewhat unstable,and the unavoidable fact that at the very most theres only 18 playing at any time.But the maps are very good (most of them) and memorable quotes from the humans ("If you can handle a piece of this I'll give ya a cookie")
Tremulous is a great game,but the aliens could use some better class variation like in Gloom.Atleast a weak flying class would make this a bit more fun,and the Adv.Dragoon could use the spikes as mines over doorways instead of the extremely inaccurate and low range bullets it uses currently.The Adv.Basilisk and the Adv.Maruader are perfect,I love the lightning and gas attacks.
PS,Human Mass Driver could use some tweaking,the shot should reach its target faster.
Over all I like Gloom more,but its undeniable that Gloom isn't very popular for lack of a good server browser.