You can test it easiest with accessories, and if you have various sets of darklight that will be your best bet
Drop on some Mnd accessories ... this is how much more you're going to cure (not a lot, but sometimes every little bit helps)
Drop on some Vit accessories ... this is how much more HP you will have (about 500), do you need it?
Drop on some Int accessories ... this is the damage boost you'd see when using Summoner
If you have some crazy PIE melded stuff... you can see just how much MP that would get you.

Now your mileage will vary based on what you need / what is helpful based on your skill/playstyle. There *IS* a best choice, with some small assumptions. If you don't run out of MP (Pie), or die from unavoidable damage (Vit), you will be best served with Mnd.

Equipment wise, Mind > Crit > Determination will give you the most healing bang for your buck. Spell speed (like skill) takes a lot to be useful, +VIT/Def only helps with damage you take.