I think crit damage might be mitigated slightly differently to regular damage in terms of physical/spell resistance. Hence the Impenetrable trait..
@Crown ?
@chevalierknight is correct with regards to the math.
Right now (until patch), Nirn gear mitigates a LOT more magic damage, though in full Impen gear the crits from tornado spammers are doing significantly less damage (when not blocking).
I've been running my old Impen gear for about half the time this past week (while leading in my more survivable builds) to see how it feels, and my survivability (vs the Nirn gear I've been wearing the past few months) is similar. Note that my normal leader build runs 50% magic mitigation even after calculating the 18% penetration from Nirn, 25% from champion points, and 10% from destro staff. That particular build does very low damage, but the point of being a leader is to stay alive and be able to make the calls for your group. A lot of opponents tend to focus me, so when leading it makes the most sense. It's not the most fun set to run (which is why recently I've been in small groups in damage builds), but it is needed at times.
Regarding stacking spell penetration vs spell crit, you have to consider how much magic resist most people will have. I've found that most TEND to run in the area of 25k (my stam DPS build runs 22k, and magic DPS build 27k), which is in the area of 40% mitigation before penetration is calculated.
If you have a Nirn staff, or two Nirn swords (as many magic damage builds go these days), then you're countering a significant amount of that resist to begin with. Add in some champion points (I'm not going to go into how broken some champion trees are right now) and you can melt the average player like a hot knife through butter.
Like everything else there's a balance to be found. If the question was oriented towards champion points, then also consider the points for extra magic damage, and extra fire/elemental damage. They can all add up (depending on what skills you're using).