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It really depends on your guild. In my old guild we had 2 10s and it very much turned into the "A team, B team" issue others are describing. However in my current guild we run 2 10s that stayed almost neck and neck on progression all the way up through H Madness.
2 10s gives you the flexibility to use people who are dedicated enough and have the time to run extra nights. We do 2 nights a week per group, and a couple weeks where we didn't have our full team for whatever reason someone from the other team would step on an alt. Obviously, this again depends on your guild and if you have people willing to put in the extra effort for that. Our GM and one of our Raid Leaders did 4 nights a week for a couple weeks straight to keep the groups at the same point in progression.
Also, personally, I hate 25mans. I like the personal accountability of 10mans and knowing the strengths and weaknesses of everyone in my raid group. I have done 25mans and they're just not for me (my computer isn't a big fan of them either). I take extreme offense to people who claim 25 is the only "real" raiding. Having done normals and heroics of both kind in Dragon Soul some fights are easier on 10 man (H Ultraxion for example) and some are easier on 25 man (H Hagara).
11 months, 3 weeks ago on A Guild Numbers Question