mp5 can be most important. but you have to balance for the highuest content you're seeing. If you're frequently on the edge for mana, then mp5 is your biggest need.
I tend to be doing raid heals, which means CH spam, in our 25 mans and I'm never close to being out of mana, so I started gemming (and in a few slots, gearing) for haste and crit, letting my SP drp down from 2400 to 2000 to pick up the other stats.
Now, if I was asked to be primary heals on the tanks, I'd probably have to excuse myself and run to the AH . . . but with a couple of holy paladins in the guild, that won't happen for me soon :)
Edited, Feb 27th 2009 10:10am by kingpatzer