Sorc: Spellbook or MMS?

I have both sets (Magic Level and Defense) for my ED level 270.

Always depends in the situation

Given Joshwa's calculations, here is my input:

With a spellbook, damage goes from 380/593 (average 486) to 394/615 (average 505). This in an average increase of 19 damage per round.
With an MMS, you block 17 damage per round. Let's do some calculations:

Assuming you hit the average SD hit in each 'mode', it will take you 4.8 rounds to kill the Hydra without a spellbook, and 4.6 rounds to kill it with a spellbook. Considering the 2 second delay on shooting SD runes, this saves you 400 MS per Hydra, or about 4 seconds for every 10 hydras (48 rounds compared to 46).

With this in mind, we must see how much that extra time saves us. A Hydra will melee once every 2 seconds, meaning its rate of damage is the same as our rate of SD. That means, for every 10 Hydras, there will be 2 extra melee hits (assuming you are face tanking them). This means that, with a shield, you take an average of 466 (233 * 2) extra damage from the increased roundage alone. However, if you consider the fact that you saved about 17 health per hit across a total of 48 hits, you actually came out with 350 extra health (48 * 17 - 466).

With all of these things considered, we can draw a conclusions:

Its actually better to use a spellbook, as you likely will not be face-tanking the Hydra. Furthermore, even if you are, the 350 health you will lose per 10 Hydras comes in exchange for using 2 fewer SD runes, saving 216 gold. Moreover, this makes the Hydras hit with fewer spell-based attacks. Though 400 MS doesn't seem like much, it has a huge effect after, say, 100 hydras (we're talking about 40 SECONDS of tanking). It is also worthy to consider that, in the case of multiple Hydras, the quicker they die, they less likely you are to get headshot. Also, more capacity is a plus (20 fewer SDs per 100 Hydras and a lighter spellbook).