Levelling as a DPS is far quicker; levelling as a tank is easier. Your choice, both are viable. The quickest way to level a Guardian is go DPS with a healer companion as you will literally never have to stop and heal. Other classes do have to regenerate their resources time and again, we don't.
According to the number crunchers, the hybrid build is the best for tanking atm:
Something like this. You can play around with some of the talents (like the quite useless lower tier ones in Vigilance: you can take Defiance and Gather Strength instead of, say, Shien form and Narrowed Focus, roughly equally useless for tanking; Stagger is also a PvP skill, really) but you get the idea. You might have some issues with Focus generation with this build so a "standard" Defense build might serve you better.
I usually use something like this. You may take the Accuracy talent in Vigilance instead of the three points in the Focus tree if you feel you don't have enough Accuracy on your gear.