Gruul
Before the Gatecrash spoilers came out, I already looked at the different color combinations (similar to what I did for Return to Ravnica). Gruul (at red-green) seems to be the weakest combination (without looking at its guild mechanic) because of its lack of evasion. Both red and green do not have abundant access to flying creatures which makes it difficult to win out games. Red does have access to first strike and double strike while green gets trample and regeneration. Compared to the four other guilds, it simply is not enough for Gruul to be a good pick.
I now look at the four other guilds through their mechanics. Dimir has Cipher, Orzhov has Extort, Boros has Battalion, and Simic has Evolve.
Dimir
Sealed decks being creature intensive, I've removed Dimir from my choices because the Cipher ability appears only in non-permanent spells. I don't put in a lot of non-permanent spells in my deck (most of the time the slots are dedicated to creature removal and other important game-breaking spells) which makes Cipher iffy as the reason for choosing the guild. One point of argument though: Dimir is a personal favorite because it is the guild back in Ravnica that allowed for milling of cards. Cipher might just come out as an add-on whereas milling is a guild-wide ability. I wouldn't be surprised if Dimir comes out as the guild of choice due to milling (I might even opt for this guild come prerelease time).
Orzhov
From the three other guilds, I remove Orzhov and the Extort ability. I've read a lot of comments on how Extort can eventually win games with the 2-life swing that it can provide. Personally, I don't want to get into a situation wherein I would have to play a lesser creature or spell (because of the lower mana cost) just to exploit the Extort ability. Granted, Extort would be useful in close games or situations wherein there are already a lot of extra mana to pay off each Extort ability. Again, Orzhov can be similar to Dimir (in terms of selecting it as the guild of choice) and I would choose Orzhov because of the mechanic and also because of the amount of creature removal that black and white can provide.
That leaves me with Boros and Simic.
Boros
Boros' Battalion ability is just what the guild needs and in sealed deck play it is easy enough to achieve. I know how good Boros was during Ravnica (there's a reason why BDW is called Boros Deck Wins) it's very fast too. But fast decks don't easily make the cut. It might just run out of steam in the endgame.
Simic
I really love Simic and the Evolve ability. It's creature-centric. It's flexible. It can be a powerhouse. And it seems to be the strongest of the four guilds' mechanics. The color combination reminds me of the Izzet color combination. I wanted to play Izzet but balked because of the guild mechanic not being able to be a big help to the guild. Simic right now reminds me of that fascination of the combination. The nice guild mechanic has helped my decision.
In conclusion, I would have to rank the guilds. First would be Simic. Even though I removed Dimir early, I'm putting them second. Third would be Orzhov. Fourth would be Boros. Last guild of choice would be Gruul.
If you do pick Gruul, tread carefully.
I'll be back once the prerelease is over to see if I picked well.
