It can be a bit weird as priest.
Int is a stronger stat (due replenishment) then spirit, but you want to keep both equal for regen.
Haste is stronger then crit, but crit can offer mana and haste from talents.
Holy concentration and Surge of light give free spells, imp holy concentration gives haste. And crits give a higher chance for overhealing and with that serendipity procs.
With 3.1 it will offer only mana with Holy concentration and Surge of light.
But then normal spell casts give haste(Reduced haste time to be precise) with serendipity.
MP5 is still the solo best form of mana regen, but you can get enough mana regen from spi, int and crit.
Spell power is a lot needed stat and wins first choice.
I would go for
Spellpower > Int = Spi > Crit
Haste and MP5 then as nice "Free" stats if gear has them
For nummers, aim for 17-20% crit during naxx 25, a bit haste, and 1000 int and spi. (i gem int and use trinkets for spi).
