I've been playing Holy since vanilla MC and @ every expansion release I have had a terrible first 2 months since holypriest without any gear (aka regen) is just terrible. Took me a long while to get used to it again but now I feel much more relaxed about my mana. You just have to give it more time to feel 'safe' as holy as well.
I've never ever liked disc so always refused to play it, atm though it is the best healing spec. It's essentially mandatory when you have to use 3 healers on some fights. Oh well, I get by with Holy, can be on par with the disc priest in my raid sometimes but usually I cannot get near him. I expect they will get nerfed sometime in the future though, so just play what you like best.