Rules of Thumb for Holy
Stack haste up to 12-14%. Think hard about whether you need more than that, given raid buffs and talents.
Pick up as much intellect as you can. It's always valuable.
Spellpower is always good.
11 spirit = 4 Mp5, ignoring spellpower gains entirely. Given spellpower gains, a 2:1 ratio is acceptable.
Crit is always valuable, although less so once you break 30% raid-buffed (diminishing returns on ilvl points spent).
Rough goals for the start of Ulduar.25 are to have 2800 spellpower, 26-28% crit, and 12% haste, along with ~ 500 Mp5 I5SR fully raid-buffed.
Rules of Thumb for Disc
Pick up haste up to ~ 11% (hard-cap). Absolutely do not stack more.
Spellpower is always good, especially since you don't have Spiritual Guidance scaling your levels.
Pick up as much intellect as possible.
11 spirit = 4 Mp5, with no spellpower gains.
Rough goals for the start of Ulduar.25 are to have 2500 spellpower, 35% crit, and 8% haste, along with ~ 500 Mp5 I5SR fully raid-buffed.
Basically, though, if you see a cloth item (or a ring/neck/etc) in Ulduar, and you are concerned about its value for Disc, take the amount of spirit on it and multiply by 4/11 to get the equivalent amount of Mp5. It works out that 50 spirit = 18 Mp5, as a baseline.
For spirit and Holy priests, you gain no more regen from a single point of spirit (in fact, slightly less due to the extra 1% scaling in the Disc tree), but you gain a tremendous amount of spellpower. The ratio remains 11:4 for regen, but for every 11 points of spirit you gain, you also gain 2.75 spellpower. Note that "gain" here is used in the overall, after buffs, sense. So for a holy priest, if you have an item with 50 spirit vs an item with 18 Mp5, the 50 spirit gains you an extra 14 spellpower, a nice buff and enough reason to choose the item.