The Seattle prerelease was about 58% Phyerxian overall, though the sealed portion was closer. 2HG was way more heavily Phyrexian, as you might guess since that effectively puts their opponent at 10 life to start instead of 30.

Certainly if you look at just MBS and go card-for-card, the Phyrexians are better. Way better. No argument there.

However, one of the things I've noticed from doing this little test repeatedly ( http://boosterme.heroku.com/choose ) is that if you go Phyrexian, it's not unusual that you get pretty much nothing from your three SOM packs. If you go Phyrexian, you're almost certainly going Infect, and it's not too hard to whiff on any decent infect cards in three random packs of SOM. We're all used to drafting SOM, in which Infect is viable. But Infect pretty much sucked in SOM Sealed unless you got a Skittles or a Putrefax.

So, if you go Phyrexian, your SOM pool can easily suck. A lot of the good-for-limited rares in SOM are completely dead if you go Phyrexian: Hoard-Smelter, Steel Hellkite (well, not dead, but certainly not synergistic at all), Chimeric Mass, Etched Champion, Precursor Golem, Spikeshot Elder, Sunblast Angel, etc.

If you go Mirran you are virtually guaranteed a bunch of decent Red and probably at least some good White; all the good RW Metalcraft stuff in SOM will complement your MBS packs. (If you go Mirran, you are almost certainly playing Red, with either W or U as your second color. Splash Black if you get multiple Go for the Throat.)

I'm sure what'll happen when I go Mirran is that I'll pop a Skittles and a Putrefax, and that when I go Phyrexian I'll pop a Koth and a Hoard-Smelter. I think people need to be ready to be screwed really hard by getting the "wrong" SOM pool for their faction, and I think the odds of this happening are substantially higher if you go Phyrexian.

That noted, as you said, the incremental card advantage is still better on the Phyrexian side, but I think bombs and synergy favor the Mirrans, so I think it'll be more even than the picture you painted.

We'll see how it goes.