I wouldn't be surprised if in some instances a paladin tank is better than a warrior tank in order to get Gold in Challenge Mode. Not sure what they'd do about it since it's supposed to be hard to do, but not being able to use another tank to get it done is bad. I do hope they don't nerf the place in those cases, but we'll see once it goes live.
There might be minor differences, but you can't just say 'I can envision a paladin being better than a warrior' because frankly unless there are 5 classes in the game and you're forced to bring one of each, of course there will be differences. The point is they're not substantially different enough in terms of capabilities for it to matter. BC is LONG gone.