IMO this depend on what gear you have. You should get crit % to reasonable value to proc your talents, after that crit is weaker then haste. my personal threshold is 25% holy crit unbuffed and i'm trying to keep it at that value. (from holy priest perspective)
This is all about personal preference. As disc you rely on damage mitigation which is directly impacted by crit heals. of course people will argue with haste you get more heals out so you have a higher chance to get a crit heal. The downside is that the more heals you are throwing out in less time you start to have mana issues.
I have my disc priest at 30% holy and 15% haste,. thats where i like it.
25% holy crit unbuffed feels good with my holy spec so I'll probably stay there. Currently at 20% haste but would like a lot more. I wouldn't gem for haste though, spellpower, spirit and int are much more attractive to me.
In the most basic terms, gearing for Holy and Disc is
Haste/Spirit for Holy
Crit/Int for Disc
But as the numerous discussion threads on here are saying, crit is good for holy, and haste is good for disc. Crit procs Divine Aegis for Disc which is one of Disc's main damage reduction bubbles, so you want enough crit to make sure that stays up.
As said, it's up to preference, some Disc Priests run with no Haste (or as little as possible) and live through borrow time, some stack Haste and replace Flash Heal with Greater Heal.
Since Spirit gives us next to nothing (other than poor regen), I tend to try to grab Int/Stam + SP/Haste/Crit pieces or SP/Haste/mp5. I have 32.6% Holy Crit and 617 (18.82%) Haste Rating, I think it's a nice balance, I'd like more Haste personally so I can try out Greater Heal properly but that's my opinion/personal choice.
If Holy is your offspec, this can effect your gear choice, however I feel you gimp yourself as Disc when wearing gear for both sides.
For Holy Priests, Haste is generally because it provides more GCDs, is vital in burst healing situations, and scales better with itemization. However, Crit cannot be ignored becasue it increase SoL procs and HC uptime. As such, I aim for 25-30% Crit raid buffed then stack Haste. I'm currently sitting at roughly 30% Crit and 25% Haste and I expect I'll end up somewhere around 32% and 28% respectively in full 264 gear. So, in general, Haste > Crit for Holy and I like lots of Haste/Spirit gear.
For Discipline, I think it's a little bit more complicated. Haste is definitely awesome up to the BT soft cap, but opinions vary after that. If you find you're doing a lot of shielding on the raid, a high BT uptime means Haste is largely wasted past the ~6% it takes to GCD cap; however, Crit also scales very poorly with shielding too since only the glyph portion can Crit. If you find you're primarily tank healing, your BT uptime will be lower and more of your casts will be longer than the GCD anyway (Penance on CD and some GHs) so Haste is still useful well past the BT soft cap; Crit also scales well since all of those abilities can crit and proc DA. Thus, I see Haste and Crit as roughly equal for Discipline, with Crit maybe having a slight edge; however, since Spirit scales poorly and is generally not needed anyway, there's no reason not to stack both with Haste/Crit gear.
Take cloth gears without spirit if you intend to go Disc main spec. The reason is simple: at this gear level, spirit becomes almost useless as a regen stat for disc, so why not drop it for haste? Even if you are primarily a raid shielding priest, you will find yourself casting actual heasl quite often, so haste after the "BT cap" wont ever be wasted.