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!)
This might work in KP and EV, but it definitely doesn't work in EC or TfB. Those instances are specifically designed such that you need 2 full tanks for pretty much every boss except for Minesweeper and Operator IX (the puzzle bosses), and, even then, you're pretty well served having 2 full tanks for each.