I am a holy Paladin whose guild is progressing through SSC and The Eye. I have always strongly valued MP5 but I am starting to see that my recent and upcoming gear upgrades (T5) are itemized to include a lot more +spell crit rating and less MP5 than the gear that got me to this point. Would you care to share your thoughts and experiences on how valuable +spell crit rating is at my stage of the game and beyond?
Spell crit and mp5 basically do the same thing: make your spells cheaper. A paladin is usually always casting, so is always getting the benefit of spell crit. Mp5 has the added benefit of working all the time, and spell crit makes your spells slightly more powerful (on average).
Now which is better is a complex question. It depends a lot on the mix of spells you cast. The more expensive the spell, the more mana spell crit will return. I'm finding that as I move up in content, I'm casting more and more Holy Light(Rank 7). It us less efficient, but I need the extra healing. So the extra spell crit on the gear helps support that playstyle.
However, mp5 still helps, especially on fights where you aren't constantly casting. So in the end, you have to maintain a balance of +healing, mp5, and spell crit. My general philosophy is upgrade whenever possible, just go with what stats you get, and if you think you are getting too low in one stat, use gems to boost that stat up a bit. I use green +heal/+int (mana + spell crit) or purple +heal/+mp5 gems usually.
Also, when you start looking at spell haste gear, spell crit will work better with spell haste than mp5 will. This is because spell crit's effect depends on the number of spells you cast, while mp5 depends on straight time. With spell haste you cast more spells in a given time, so the amount of mana returned by spell crit increases, but the amount of mana returned by mp5 stays constant. So if you're planning on picking up any spell haste Badge, ZA, or crafted gear, I would start leaning towards spell crit more heavily.