Potency of "Instant" spells

[Therin]

I have the option of using either Instant Stoneskin or Instant Phalanx. Obviously Instant Phalanx should be more damage mitigated over time, but I was wondering exactly how Stoneskin would add up to it.

Also, would a MND+25 plate enhance these traits like it does for players, or no?

I could also use VIT+25 but I'm not sure how effective that would be versus Stoneskin or Phalanx.

[Halion]

Instant spells only last a certain amount of time which is in all seemingness fixed despite outside stats at the level of the ability itself. Blink being the most obvious example as it gives another shadow. For Phalanx it's hard to judge exactly how effective it is and how long it lasts, but it seems to pair best with an already defense mob that has a Defense Bonus and/or a Damage Resistance Bonus. I would imagine it lasts as long as Instant Spikes do. It also depends on what your mob is trying to do. Stoneskin pairs great with a caster that will bust out a high damage spell out of the gate because it stops them from getting interrupted. Stoneskin also guarantees a fixed amount of damage reduction so if you have a high damage mob whose winnable fights tend to not last long then you will get more out of SS than Phalanx.