Aye I guessed but what stacks better for a disc priest? Spirit or MP5?
Right now it's kind of a wash... MP5 is better per itemization point while inside the five second rule - where we usually are - but spirit does give more return during the phase shifts common in many ulduar fights (heart phase of XT, transitions in mimiron, etc). After 3.2, where you get more MP5 out of the itemization budget, MP5 will be better but you probably should still just take MP5/spirit on gear as it comes, rather than specifically seeking to build that stat.
For leveling spirit is still good for running from mob to mob. I would try to balance out Int and Spr while leveling. Sta is always good too for leveling.
As for a Disc leveling spec -> * Disc LvL Spec *... I hear people say that Spirit Tap sucks but I can't see how really if you just get a bunch of Spr, might not be what it used to be but it's still a boost in regen for lvling. I also haven't leveled in a really long time though too lol. try and fill the Shadow+Holy talents first, they will help fights go faster, then the Disc tree just makes it easier after that. The LVL 80 healing Disc spec isn't much different than that, just minus the shadow and switch around a few other talents and boom you're a raid healer.
Int Int Int...don't even look to go after spirit or MP5, just take it on items that are upgrades. Gem spellpower+int when you can finally gem. With Hymn+Fiend, Int is really all you need in the way of regen.
When you are actually healing heroics and raids, you want to reach ~25% crit before buffs, and around 12-13% haste. Those are just goals to build up to. You can do with lower haste though since class buffs take care of it easily but it's still good to have in case a buff is missing or you're solo or you don't have Borrowed Time up.
With that said you can see haste isn't too important as Disc because between Enlightenment, Borrowed Time, and other class haste buffs, you'll reach the 1sec GCD if you have ~4-5% gear haste.
If you want to have a low GCD and cast time without raid buffs then you need ~13% haste from gear, because that along with Enlightenment and Borrowed Time will bring you to the haste soft cap of 1sec GCD.
So focus on Spellpower and Int, but try to hit the goal of at least 25% crit - 12-13% haste unbuffed at the same time.
If you enjoy Disc stay Disc. It's very viable in raids and loads of fun.
You guys would be pretty damn surprised how fast Disc can grind. Reflective shield is awesome. PI+Shield+Borrowed Time+Holy Nova and you can pull huge packs over and over. It doesn't get that good for a little while though, but it's good to play what you plan to stick with I think. Then again it's always good to see the other side of your class, so leveling shadow can be fun too. To each his own.
I think Disc owns at 5 and 10 mans more than Holy too. Better efficiency, and since it's only 1 group, it can still PoH and shield spam easily. PoH is 25% hasted every time also. Holy really only shines in 25 mans. That's where it's really good. Both work though no matter what of course and all that matters is what you find fun
I also already pretty much pinpointed lvl 80 Disc stats in my previous post.
I was either #2 or #3 fastest leveled priest on my server, and I was in my Disco/Holy leveling spec. I never, ever ran out of mana and I chain pulled everything. No, I did not stay up for 48 hours straight. OFC, this was before the Spirit nerf and I had spirit stacked out the ying-yang in my lvl70 holy gear.
Shadow is great for leveling as well, don't get me wrong. But if you find disc/holy more fun, you aren't sacrificing much, if anything at all, to be in a build you like more. Make sure to grab Spirit Tap first, then the holy talents, then the disc talents.