MagP vs AP/LV Glyphs On Rammus

So i have gotten into Rammus recently and i like em, I don't run MR/LV or flat MR glyphs on 'em because if i'm playing Ram, the enemy team doesn't have much AP to begin with and i can easily get MR items later in the game if an enemy AP champ gets fed.

I run either 9 AP/LV glyphs which is 1.53 AP/LV and totals 28 AP at LV 18, or... 9 MagP glyphs which gives me 5.7 MagP.

5.7 MagP for a late game 28 AP...

The thing is Ram's ult deals:

MAGIC DAMAGE PER SECOND: 65 / 130 / 195 (+ 30% AP) MAXIMUM MAGIC DAMAGE: 520 / 1040 / 1560 (+ 240% AP)

30% AP, or a total of 240% AP (considering i get to hit them with the entirety of my ult) is not that great on Rammus. The reason i say this is because he doesn't get much AP items; if you aren't putting at least 3 tank items into Ram then you not playing him right IMO. So 28 AP turns to ~8 bonus dmg/sec. or a maximum of 64 bonus dmg. Not that much.

The way i see it, his Ult already deals amazing damage on it's own, i mean 1560 total AoE mag. dmg at lv 16 is crazy, all i would need is MagP to make sure most of that dmg hits. Also, the MagP would be working from the start for the Q's and once is hit 6 where the AP/LV would only be giving me 1.53 AP/LV which hardly makes any difference both early game and late game.

What do you guys think????????