At the moment I have a level 40 Guardian that is my main character. I am in the Focus spec and so far I am doing a ton of damage in warzones and flashpoints. You can switch between specs on the Republic Fleet. I almost always stay in Focus, but sometimes switch to defense for raids. Whichever you choose I recommend the Guardian over the Sentinel. Hope you take the fact that Guardians can play as multiple roles into consideration.
Ericmor