The most popular (and generally most effective) choices are +4 WIT/+4 CON (-STR)/+4 DEX (-STR) and +5 WIT/+4 CON (-STR).
Don't worry about taking a few points from MEN; the difference is negiligible, and the boost to casting speed from the extra WIT is significant.
The most noticeable difference from +4DEX is +5 run speed, though of course this boosts your accuracy, evasion, and atk speed slightly (not usually of concern to SEs). Supposedly increasing DEX increases your shield block rate, but I don't know by how much.
Going +5 WIT instead of +4 DEX is a perfectly viable option and better for some situations. No amount of +WIT is going to make your heals very good, and your mdef and mp regen will suffer just slightly more for this change, but if casting as quickly as possible makes you happy, you might enjoy one more WIT point over a DEX boost.