So you think you can heal:

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effective mana formula(partial): mana(1 + (.60 * crit%)(1+(.60 * crit%)(1+(.60 * crit%))))
an explanation:  The problem with most effective mana formulas is that they assume that after crit mana regen they don’t assume that the regened mana can benefit from an additional crit.  That is why the same thing is calculated several times.  More can be added to the formula by repeating the (1+(.60 * crit%)) more times but beyond 3-4 times isn’t significant in the least.

Additionally you can take the effective mana formula and add the accumulated mana from mp5 based on the length of a fight.

Full effective mana with calculating mp5 on a given fight length: (mana +  fight length in seconds/5 * mp5)(1+(.60 * crit%)(1+(.60 * crit%)(1+(.60 * crit%))))

mp5 vs crit:

assume 100 casts of flash of light with 1% crit

100 casts = 150 seconds casting = 30 ticks of mp5

100 casts with 1% crit = 1 crit regenerating 108 mana over the 150 sec fight

108 mana / 30 (ticks of mp5) = 3.6mp5

using Holy Light instead: 1% crit again

assuming 100 casts of holy light (impossible, but it’s theorycrafting) with 1% crit

100 casts = 200 seconds of casting (assuming  Light’s Grace was up first) = 40 ticks of mp5

100 casts with 1% crit = 1 crit regenerating 504 mana over 200 secs

504 mana / 40 = 12.6mp5

Haste:

Haste helps improve healing per second by allowing you to cast more spells in a given amount of time or generally speeding up each cast slightly but by casting more you are using more mana.

0% haste casting 100 (150 seconds) Flashes of Light with 20% holy crit (reasonable).
180 mana * 100 casts = 18000 mana used. 
20 crits * 108 man regenerated each = 2160 mana regenerated.
485.5 * 80 + 485.5 * 1.5 * 20 = 38840 + 14565 = 53505 total healing

20% haste casting 120 (150 seconds) Flashes of Light with 20% holy crit.
180 mana * 120 casts = 21600 mana used.
120 * .2 = 24 crits @ 108 mana regenerated per crit = 2592 mana regenerated
485.5 * 96 + 485.5 * 1.5 * 24 = 46608 + 17478 = 64086 total healing

1% haste = About 1% more healing (~.99%) and 1% more mana used.

Conclusions:

When wondering if you should stack mp5 or crit the simple answer is both.  Mp5 ticks no matter what while crit regen (illumination) only “ticks” when you cast a spell that crits, so someone stacking mp5 will be up and casting sooner.  Someone with decent amounts of both will be able to show decent amounts of regen while casting without relying on crits but still have crits to basically be a “bonus” with mp5.

Effective mana (<crit loadout> 32% crit using gruul fight length from 5/20 (5 mins 38 secs) and 130 mp5): (14077+ 338/5 * 130)(1+(.60 *.32)(1+(.60 *.32)(1+(.60 * .32)))) = 28259 effective mana

Effective mana(<mp5 loadout> 26% crit with same fight length and 200mp5): (13177+338/5 * 200)(1+(.60 *.26)(1+(.60 *.26)(1+(.60 * .26)))) = 31612 effective mana

Regarding haste: Haste improves healing output per second but doesn’t improve healing per mana so while you may heal more in a given amount of time you aren’t healing any more efficiently.