Gotcha, well some champions (using all abilities) go well over 100% utilization of AP, but we'll do that just because.... simplicity and to get a base line.
Morrelos and void offers 70 AP for 140 total. Sorc obviously offers none.
So we have the 40%/+30
At 140 AP, I think it's obvious having the pen is better at ALL MRs so lets ignore that.
FYI MR/ARM is calculated by
Resist/(Resist+100)
So for 100 MR 50/50+100 = 33%
If the pen items are all a person has then.
Resist -(void) -flat = damage done
50 MR -(20) -30 = 140 damage, True damage
100 MR -(40)-30 = 107.7 damage
200 MR -(80)-30 = 73
Lets start low at 200 AP (No pen)
50 MR = 200-33% = 133
100 MR = 200-50% = 100
200 MR = 200-66% = 67
So at 200 AP you are less effective than a full pen build at all times despite having less AP.
The break point however is not far.
At 225 AP we are already more effective than a full pen build.
50 MR = 225-33% = 151
100 MR = 225-50% = 112.5
200 MR = 225-66% = 76.5
While 225 does instantly surpass a full pen build, the only factors to truly consider are cost/AP ratios. If your AP ratios are lower than 100% combined on a rotation than you must have even more AP than this to break even. (and get that juicy +3.5 damage on a target with 200 MR)
Cost will bite you though, Sorc immediately make it to where getting three other items is just worse.
And then getting a Rabs or something AP heavy on the pen build makes the numbers skew heavily to the pen build.
Lets do that in the 225 AP vs 140 AP +Pen and add Rabs.
140AP+ Rabs = 364 (PEN)
50 MR -(20) -30 = 364 damage, True damage
100 MR -(40)-30 = 280 damage
200 MR -(80)-30 = 191.57
225AP+ Rabs = 483
50 MR = 483-33% = 323.6
100 MR = 483-50% = 241.5
200 MR = 483-66% = 164.22
So in general while you may get +11 damage on a 3 item full ap build against pen at 225 AP. It's probably still better to get void since afterwards the Pen build still wins vs AP even despite having 120 more at ~4 items worth of AP at all stages.
This is by no means law for every game, a 50 MR target is most effectively taken care of with 2 flat items and a bunch of AP. I was just putting them against each other with out crossing. It would get way to complicated.