I switch with my current team based on fight mechanics, but most of the time I drive with the tank, I find like Khatovar mentioned you pretty much have full control over your healer with repeater regions anyway.

The only time I'll drive with my healer is if ranged need to constantly move in a fight and melee can pretty much stay put. Although you can still do this with isboxer and driving with your tank.

As for DK versus Warrior as tank. I'm starting a new team with a DK and I've had a warrior in the past. I'd say that the warrior has a lot more mobility with charge, heroic leap and intervene. The DK does have death grip though which can be nice against ranged mobs when you AOE.

Are you going to run both the warrior and dk in this team and just decide which will dps and which will tank? if so try em both out as a tank while you level and decide as you go.