Depends on your preferences for mfing.
Pure blizz is most optimal clear wise. But you will be limited to areas without immunes or very few immunes for your Merc to kill.
Hybrid let's you do more (if you want more variety) but you'll be slower at clearing.
Personally pure blizz is enough for me right now (meph Andy, sometimes cows countess summoned pindle) But as I ween off mfing meph Andy I might want to transition towards a lite sorc for keys farming. Who knows.