Slight change of plans
Well, here I thought I had my plan of development running. Three PVP characters. Shaman solo as a DPS spec, Druid paired with my daughter as a Tank spec, and because I found I couldn’t let go, a priest that would go solo – so shadow till I could group consistently and if necessary change to healpriest. That was in order of amount of time I expected to apply.
Ha.
My daughter got with some friends and did a “wouldn’t it be cool to group all the way from bottom to top – never having to lfg for 5-mans or nasty quests?” Yep. Three DPSers, and my daughter decided she really, really wanted to pally tank. As a reminder, she’s an outstanding healadin on her PVP 70 – always in demand not only in her own guild but by a few others with whom she’s worked. “Wanna do SSC/Gruul/etc?” yeah, that good. (probably better than me). ANYWAY… pally like I’m a priest – knowing it really well despite primarily doing one spec.
So they needed a healer. And so she and I, we had this little talk. I have three potential healspecs on line, and I could always go pally if she wanted. Nope, in her experience (and both our opinions) the best all-purpose healer for getting up there is the priest. (I know there are some outstanding tools in the druid tree, but… she’s my tank, I’m her healer, we will be mutually supportive – and both of us are just biased in favor of the priest healer.)
I really considered dropping my poor pre-10 priest. I mean, I’m talking teenage boys, they may be bothered by me playing a female toon. (Three horde, one female, two male. shrug). I decided that if I wasn’t going to complain about the stupid undead (see other bias posts) warlock, they could live with my female priest. Besides… the punnnnn. So Ipallida will run higher in my playtime. And I’ll do a little learning, because I’ve never focused on being a healpriest from the beginning. hmmm, disc or holy…
This could be educational. As well as fun.
Priestly Endeavors - a gaming blog 
holy or disc? The answer is 42 😉
I leveled to 62/63 as full holy and *hardly* struggled because I built great relationships with many tanks/dps along the way. I am now something like 28/33/0 and LOVE my fully buffed +-600 MP5 and over 350mp5 while casting (I have some Kara gear).
for your situation:
1. never played Holy from bottom,
2. have pre-made group (no LFG, no solo’ing quests)
I suggest Hundreds Holy
Hi there,
I did something similar to what you did, though less involved; my brother and I leveled two toons together–he a feral druid and me a priest. The logic was much the same: We figured we’d have the two hardest parts of any group figured out for any instance we wanted to run.
That said, I leveled as Shadow until probably around level 60 when people started complaining they’d rather have a “real healer.” That was a pretty stupid complaint since I was still capable of healing instances that way at that level, but I went holy at that point. (Don’t get me wrong; while in instances I was a healer, not dropping any DPS and only the occasional VE or VT for a boss.) I suppose the point being, for leveling purposes, you can do it shadow if you want for a looong time and still be a capable healer.
That said, you’re in a different situation. With mine, my brother could really use the DPS help I was able to (better) provide with a shadow spec. If you’re going to be leveling all the way to 70 in a group of five though, your offensive contributions aren’t nearly as needed.
So, assuming you’re not going the shadow route… disc or holy? Personally, I don’t see a lot of great value in the holy tree (outside the first two points into avoiding interrupt of course) for quite some time. Blessed recovery is eh at 15 points in. You don’t really get to the meat until spiritual guidance (though possibly not worth it at low levels) and eventually spiritual healing. I see whole-group value to, say, Divine Spirit in the disc tree though–it helps you (clearly), the warlock you mentioned, and even the pally tank to some degree.
So to stop blathering and get to it, I’d dump the two points into holy for the interruption right away, then zip down disc at least until divine spirit and maybe even improved divine. At that point you will probably have a good idea whether or not your healing talents need a little help and can either stick disc for a while or zip down holy. Naturally at the top end you’re going to end up respeccing a more full holy build, but that’s a ways down the road.