Spell crit isn't a good stat to gem for in PVP, you want pure haste so you can actually get frostbolts off and get 5 ice lances on a deep frozen target with icy veins up. Haste haste haste, you have 50% chance to crit on frozen targets plus your base crit. Gemming, gearing for crit is overkill and there are more important stats. Whenever there is a choice between a crit piece and a haste piece, choose the haste one (ie: PVP neck, cloak, etc..)
Spell penetration is good for fighting classes with resistance to your spells. Typical frost mage spec would give them 110 resistance to all spells (assuming mage armor is active). Arcane mages might run with 140 resist to all spells. Paladins with frost aura get 130. Not sure if felhunters still have their insane magic resistance like they did in TBC.
Put 35 spell penetration on your cloak, then think about gemming 4x 20 spell pen gems. That will make enemy frost mages unable to resist your spells. Another gem will make paladins unable to resist with frost aura up. And another half gem on top of that (ie: 8 haste 10 spell pen in a yellow socket) would make enemy arcane mages unable to resist and you'd have maximum spell penetration for PVP (145). It's up to you though.
Top mages have very high arena ratings, get tons of arena points per week, and have multiple PVP weapons and off hands. Against caster teams they'll swap into their spell penetration set, you can even swap your weapons in combat and in arena (at the cost of a GCD but it's no biggie).
The problem with throwing 4-6 spell penetration gems in your gear is it's useless against alot of classes, just a waste of stats. But duel enough frost mages and have counterspell resist often enough you just get fed up and start gearing for spell penetration.