Well, i've only tried to level a priest and druid past 70 as holy/resto (my pally is prot, and will be staying that way forever, thankyou), and i honestly found the druid easier. My priest just seemed to oom so much faster. I suspect that was, however, because of the fact that he was shadow pre-wotlk, so wasnt really sitting on top of that much mana regen; my druid was resto, and so had plenty to go around.

I also actually liked the flavour of resto levelling more... starfire/moonfire/wrath, chuck on a lifebloom when youre almost done, rinse and repeat... or maybe i just couldnt adjust from melting faces with mindflay on my priest ;)

something tells me that it will probably come down to whatever youre most comfortable with, because they all obviously have different playstyles, and youre bound to find one class a little bit more fun than others. resto druid did that for me. maybe you like totems, or plate, or smite more. IMO, the style that you enjoy most is also going to be the fastest levelling for you, because as they say, time flies when youre having fun :)

Btw, i feel the same way as you... i played dps classes mostly, right up until i specced resto on my druid in outland, and then fell in love with healing... now i want one of each! :)