I prefer guardian over sentinel (have a level 50 sentinel, level 30 guardian).
I much prefer the animations for a single lightsaber, and force pushing someone off a cliff never gets old.
DPS-wise there doesn't seem to be a noticeable difference - you'll still destroy things quickly as any DPS should.
And of course, if you want a change of pace you can always switch to a tank spec.
Additionally, I think the orange level 20 legacy armour for the guardian is the best looking armour in the game.