If you're just looking at BGs, I would (not to sound condescending by any means) not matter so much with Spell pen and what-not because as it usually is; 7v3 at a Arathi Basin base = Stamina and Spell Power > Spell Pen. Though, I'm not sure if you actually wanna build a set for -just- Battlegrounds.
Arenas are an entirely different monster. Spell Pen is very important here. Minimum to get would be about 75 with a "cap" at 135. If you're only gonna be in gear that you have bought with honor and what-not I'd go with about 110 spell pen. Reason being, mages are **** with all of their spell resist. Having a Wind Shear resist and not being able to lock a mage out of their Arcane tree only to be poly'd and counter spelled into oblivion really sucks.
So, I guess I'm saying in regards to battleground: Go with SP\Sta unless you are really serious into min\maxing for an unpredictable circumstance. Which, is obviously hard to do.
In regards to Arenas: Get 110 Spell Pen then move on to SP\Sta. Once you have some ranking and get into the relentless gear, move up to 135 Spell Pen unless you're seeing an absurd number of resists at lower arena ranks. Then, start off with the 135 spell pen.
I used to be a skeptic until my druid saw chain resists on cyclones. Spell Pen then caught a new light in my eye. Needless to say, it's very important.
My two cents.