Either would be fine, seriously. Both Vanguard and Guardian also have increased DPS builds that are suitable for off-tanks but not main tanks, so if your main tank is a Shadow using one of them as an off-tank to squeeze a little more DPS out of the group works very well. (Trying to go hybrid with a Shadow is pure disaster!)
If you want someone to tank and DPS in the same spec, do not pick a guardian. Guardian tank DPS is seriously about half of what the other two tanks can do.