Disclaimer: This post contains theorycrafting. Chances are, I am completely wrong but bear with me anyway. If numbers scare you, turn away now.
To simplify my analysis, I have considered the first case of paladin healing as mentioned in an earlier post (here) In each case, the player has a latency of 0 ms, and the fight lasts five minutes, the latter of which is entirely plausible.
The player spams max rank Flash of Light throughout the duration of the fight. In five minutes, he would cast 200 FoL's. With a 1% chance to crit, he would crit 2 times, and gain 216 mana from Illumination. 1 mp5 would only return 60 mana over the 5 minutes period. Before the Illumination nerf, the returns from crit would be even more: 360 mana returned.
Clearly, then, crit chance is a more useful stat, even after the illumination nerf. Question answered? Problem solved?
If only it was that simple. Now, I don't have official, or even unofficial, word on this, but it is glaringly obvious, that spell crit requires a bigger item budget than mp5. In fact, you'll rarely find an item with more than 2% chance to crit (at level 70), while even 10 mp5 is not that uncommon.
Quick reanalysis. A 2% chance to crit would only return 432 mana (on average) over 5 minutes, compared to 600 from 10 mp5. However, with the old Illumination, it'd have been 720 from crits compared to the 600 from mp5. Goes to show the harshness of the nerf.
All this is further complicated in that crits also increases your healing done, which can be significant against certain mobs (read: heroics). Add in a crit while you have Divine Illumination, and that tilts the scale again. Divine Favor does not count because it is not affected by crit chance, it is a guaranteed crit no matter what.
Ultimately, coming up with a metric to compare these 2 stats is nigh impossible. It all comes down to preference, and you much of each you have already got. If you are ending fights at 80% mana, maybe it is time to wear that Critmasters Amulet of Spell Crit, and rank up higher on the healing charts. And make sure you don't ignore Stamina or Intellect either. If you are getting one-shotted by Aran's pyroblast, you are useless no matter what.