Pffff. You can do it all as Holy, you know. Don't let the Disc svengali fool you.
Okay, maybe not arena.
:)
I agree that Disc is best for what you want to do. You can do heroics and BG's as either, but for any serious PVP you want Disc, hands down. You can also give both a try and see which you like, as the play styles are different.
Gear-wise, for PVP you'll want some hit anyway. Once you hit 80 you'll start picking up gear for your main spec and I don't think the greens/blues you get there with will matter much. My personal opinion is that Spellpower is always a priority, while Spirit and Crit carry a bit more weight with Holy and Int and Haste a bit more with Disc. But all five stats matter for both builds, and balance is always important. I wouldn't, for example, stack a ton of Spirit at the expense of a 14k mana pool even if I was Holy.
As for leveling, I agree that if you can afford dual spec, it's a good idea to buy it now. That way you can keep your Shadow build but switch to Disc once in a while so you can get used to it and will know it well at 80, and so you can heal instances on the way up with the same tools you'll be using at 80. I just feel like it's nice to hit the ground running at the level cap, since learning a different talent tree really can be a big change.
If you do switch to Disc (or Holy) while leveling, I highly recommend keeping Spirit Tap in your build. It just makes things a billion times quicker (especially if you're picking up healing gear which tends to have spirit instead of hit). I leveled 70-80 as Holy but with a Spirit Tap build and I didn't even keep drinks in my bag.
Edit: I forgot to say that if you don't buy dual spec, I'd actually just go Disc whenever you want. Disc is really not a slow way to level. With Reflective Shield and AOE damage Disc's efficiency against multiple mobs makes Shadow, IMO, only marginally faster.
Edited, Jul 26th 2009 8:52am by teacake