Enmity across the 2 classes is actually very close. The only major differences is how durable each class is, and how much DPS they dish out (the higher DPS rotation of WAR's is about the same in terms of enmity as the higher enmity rotation of PLD).

What it all boils down to is will there be a niche for both classes? Everything that makes a good main tank also makes a good off tank. So having one class relegated to the lesser role (off tank) while the other can effectively do both would be a bad situation.