What should a holy priest gem for?

Answer 1

It would depend on your progression, gear level and personal needs. At the lower levels (Naxx/Ulduar) most Priests would gem for Spirit/Spellpower. This increases their throughput and mana regen at the same time.

But at the high level, at Icecrown Citadel, the gemming becomes almost exclusively Spellpower, Spellpower/Crit or Spellpower/Haste unless the Spellpower/Spirit gems are needed to activate the Metagem in helm, or to gain a +9 spellpower socket bonus.

The Crit gives bigger heals, the Haste faster heals. In this case it all increases your healing output, and a balanced value between the two seems to be best (within about 10-15% of each other).

Holy Priests do not have any talents besides Inspiration that activates on a healing Crit (even though healing crits are always nice) but many that rely on casting spells in sequence (such as Holy Concentration and Serendipity) so Spellpower/Haste would be most beneficial for that if you have to choose in your gemming.