Tremere is better from an allround perspective, not to mention better in tournaments.
Gangrel is just plain good at killing stuff dead.
I'm going to advice against starting with the Tremere from Keepers of the Traditions. They suck.
Well, not suck. But they're vastly inferior to the oldschool starter (which isn't impossible to get, even though it's out of print. At least not in my experience).
Consider instead the new Tremere Antitribu. The G4 !Tremere are ridiciously good. And so is the starter.
Malgorzata and Orlando Oriundus, to be precise. Splash with Ladislas Toth, The Torch and you got yourself a nasty piece of deck with plenty of vote defense, anti-stealth defense (Auspex) plenty of offesive power (They got the games single most powerful discipline after all, Dominate) and the toolbox/toubleshooter discipline of all times, Thaumathurgy. I'd be suprised if there's any situation that Thamathurgy can't handle, the only drawback that in some cases you'll need to use up a fair amount of cards.
Not sayin' that Gangrel are boring, but they can do pretty much only one thing, which might be boring after a while.
And in serious tournaments, they're the underdog (no pun intended) unless your metagame differs from mine.