Kumo answered your first question nicely.

WAR is the most versatile and self-sufficient of the three. It is also the hardest to master imo due to its mechanical gameplay.

PLD is the safest option. Simplest to pick up and master.

DRK is great. Not as safe as PLD but safer than WAR. DRK takes more physical damage than either other tank but also takes significantly less magical damage than either of the other two.

There is a stigma with DRK being squishy and that WAR players are bad. This is largely due to players that never tanked playing DRK and some WAR players trying to tank in their DPS stance without using CDs.

Pre-3.0 statistics said PLD was the second most played class out of all classes after WHM. Based on that I assume PLD is still the most played tank, followed by DRK then WAR. I would guess WAR is the least popular because of its aesthetics.