I spec'd vig and def until I got the vig skill "unremitting", then I stayed in the def tree. I never had enough points to get the top skill in def: "guardian slash", but I still made out just fine.
Either way you should be fine. If you stay mostly def, you should easily be able to keep Kira all the way through, which should make things go faster compared to some of the other companions. Honestly I haven't experimented with the two later comps.
Some say Guardians are one of the tougher characters to level with, but if you keep your gear and your companion's gear up to date you should be just fine. I never needed help on any of my class quests.
--Slight Spoiler--
I spent a good portion of time on the last major planet (Corellia), grabbing gear for T7 (some off the GTN) as I had heard that he was the only comp you could take in the final mission. This proved to be a good use of time and just having him in good greens made things a breeze in the final mission.
