I am not sure if we have enough information to make a full decision about this but here goes my thoughts on it.
First of all if you have not run a close knit 10 man it will be hard for you to understand just how awesome they are.
A 10 man is usually going to be a close group of friends that have been playing together for some time. This makes for a lot less QQ’ing about loot and progression. Nerd Rage is hard to deal with for the person raging and the people getting raged on, in a 10 man that is curbed quite a bit. Because there is less of every class it should be easier to gear, and the format for handing out the gear will be less strict because generally it is just a simple need before greed MS/OS situation.
Another bonus of 10 mans is that I can usually find 9 people I want to hang with for long periods of time, with 25 mans that has never been the case. Too many personalities, to many bads, people not doing their job and either no one wants to tell them or they do not listen when told. The drama in 25 mans is unsurpassed by anything I have encountered in my real life drama and I play a game to have FUN and ignore my RL drama even if for only a few hours.
The downside of turning to 10 mans remains to be seen as far as loot and progression. I can tell you that it is highly likely your guild will get more cliquey with each 10 man only wanting to hang with the people from their 10 man. If you have more then 1 10 man running it will become a competition of who is beating who in progression. The 10 man team that is doing better will be inundated with requests from members to join their 10 man which causes hurt feelings.
I speak from being on both sides of the 10man, having a team running that had no room and running a team that had no room. It can be hard for everyone involved, and causes a huge amount fighting and behind the back talking.
I personally only want to run 10 mans but I love my guild and the people in it so if as a guild we decide to run 25 mans I will do it (maybe happily). I am concerned that this will cause a rift in many guilds and that the backlash will leave many guilds disbanded. Planning for the raid lockouts and what kind of guild you want to be should begin now.
Current guild situation we have a bloated roster due to many “core raiders” having stepped down from raiding until Cata goes live. We have recruited many people I personally find less then friendly which is of course is part of the problem of coming to the “end” of an expansion, you end up with less quality recruits. I was a core raider that had to step down due to work, and now I am back and just floating, I look at the current roster and think how can we run 25 man when we have 35-45 raiders. I am personally unwilling to sit and wait list forever and will most definitely want to experience the new content a.s.a.p.
So what is the answer, do you drop the people who are newer, that does not seem fair. Do you tell core raiders who stepped down tough, you stepped down we are going with the newer peeps and rick losing those people? Do you try and recruit more people and run 2 25’s?
So far we have not come to a decision on what to do. There was a suggestion to run 1 25 and 1 10, but I still believe this will call some inner fighting and nastiness, yet it seems to be the best and most fair decision.
Question is how do you split the raiders up? Going to need 2 tanks min for each raid not a lot of guilds running around with 4-6 geared tanks. You need at least 3 Heals for the 10 man and 5-6 for 25 man so 8-10 geared healers. Now some would think deeps is deeps but you need to split em up right, we need both physical and magic as well as certain buffs from certain classes. It is a nightmare just thinking the whole thing through logistically.
Personally I am uber afraid as far as the guild drama that will ensue and we all start to reach 85, maybe it will be as easy as he who burns the content fastest is king (BAH)? I dunno but for now I have taken a very wth attitude about it in hopes I will be able to raid with the guildies I know and love in the next expansion.