Guardians are a harder tank to learn to play because they've got a more demanding resource system. They've also got the worst threat generation and damage of all of the tanks so, if you're planning on doing group content, you're pretty much guaranteed to have threat problems until you get geared *and* experienced.
VGs are the de facto simplest tanks to play. As long as you keep your ammo above 8, you just keep your 3 CD abilities on CD (Stockstrike, HiB, and Energy Blast) and spam Ion Pulse (if above 9 ammo) and Hammer Shot (if below 9 ammo).
As for leveling, VGs bloom way earlier than Guardians do (Guardian tanks are a pain at low level thanks to Soresu crippling your resource generation and the deep Defense talents being required to offset it), not to mention they have *way* better AoE which they get pretty much right out of the gate which makes leveling a breeze.
If you're looking for a class to learn the ins and outs of tanking with without much complication otherwise, VG is probably your best bet: it's way simpler, which allows you to focus on learning tanking rather than trying to learn how to play a Guardian tank *as well as* learning the fundamentals of tanking itself.
Also, there is actually a Tanking Forum
where you can get a lot of good information (and where this question or a variation upon it generally gets asked at least once per month with comprehensive answers). It's in Classes>Roles>Tanking.