I've cleared, Nexus, HoL, Gundrak, UK, Start, DK (all heroic) with my warrior before I swith to a Paladin because my guild wanted a paladin tank. Tanking with the warrior is just as easy to do as with a paladin. Actually I think it's more fun to tank with the warrior, whilst playing the paladin I always miss charge. As things are now the only thing you should consider when choosing a tank is what kind of buffs you want/need.

At the end of the day I feel that the instances go faster with the warrior and there is more micro management with the paladin.