math time!

assumptions 1:

1) you're casting rank 5 disintegrate.

2) your opponent has 50 magic resist.

3) you have sorc shoes and 100 ap before runes.

4) you have the 15% magic pen mastery.

so without quints, you're doing (245 + .7*100)/(1+(50-20)*.85/100) damage... which comes out to 250.996 damage

with ap quints, you're doing (245 + .7*114.85)/(1+(50-20)*.85/100) damage... which comes out to 259.279 damage

with mpen quints, you're doing (245 + .7*100)/(1+(50-20-5.67)*.85/100) damage, which comes out to 261.02 damage

assumptions 2:

1) you're casting rank 1 disintegrate

2) your opponent has 30 magic resist

3) you have zero ap before runes

4) you have the 15% mpen mastery

without runes, you're doing 85/(1 + 30*.85/100) damage, which comes out to 67.729 damage

with ap quints, you're doing (85 + .7*14.85)/(1+30*.85/100) damage, which comes out to

76.012 damage

with mpen quints, you're doing 85/(1+(30-5.67)*.85/100, which comes out to 70.434 damage

assumptions 3:

1) you're casting rank 5 disintegrate.

2) your opponent has 100 magic resist.

3) you have sorc shoes and 500 ap before runes.

4) you have the 15% magic pen mastery and a void staff for 40% more magic penetration.

with no runes, you're doing (245+500*.7)/(1+(100-20)*.85*.6/100) damage, which comes out to 422.585 damage.

with ap quints, you're doing (245+514.85*.7)/(1+(100-20)*.85*.6/100) damage, which comes out to 429.968 damage

with mpen quints, you're doing (245+500*.7)/(1+(100-20-5.87)*.85*.6/100), which comes out to 431.765 damage

...

you can disagree with my math assumptions, there could be factors pushing it in one direction or the other (for example, you're probably using mpen marks, but at the same time, your opponents might be using magic resist glyphs or masteries. differences should be minor)

Note that magic pen is relatively better for the bear as it does higher base damage with the same ap ratio, but the difference isn't that great. Mpen will also have a larger advantage late game if you never take void staff. (or a worse advantage mid-game if you rush void staff, but who rushes void staff?)

**TL;DR:** AP quints are better at level 1. mpen quints are better from level 9+. But there's not a huge difference.

i'd probably go with the ap quints myself, with a better early game you tend to snowball better, ap quints are better for last hitting, etc, and doing 2 damage less on disintigrate mid and late game is not going to kill you.