If you're purely mfing meph.. dont even bother with hydras.. by the time you get enough hydras up to do damage, you could have dropped him in 2 shots with blizzard.
Go with pure blizzard or orb.
and if you want all around pvm, buy an infinity when you get the FG then go for a chain light sorc

I once made an orb sorc to run ancients, but later it turned out that with blizzard i could clear ancients much faster. And with blizzard i have the added bonus that i can easily clear ancients in full games as well.

I came to the same conclusion:

- for boss killing FO sorc is better because of survival capabilities (talking about 3-4 mana teleport, max telekinesis and whatnot) that makes you almost totally merc independent (you will have enough mana to escape, and can take several hits) and you can put a weapon with either slow effect or Reaper's Toll in the merc's hand which debuffs make boss kills easier than ever before

- for tunnel/mausoleum farming blizz is superior because the less dangerous places let you focus on extreme rapid killing by pumping up all synergies and using insight on merc

Of course both builds are very versatile in both situations but still one is better than other by a great margain if mastered.