I think you pretty much listed some of the good and bad points right there :)
I think that whether or not this particular co-op is a good fit for you and your family is going to depend on a few different things.
How involved do you really want to be?
A co-op will require a lot more participation and work from you and your family than a regular preschool. Now- this can be a great thing, because you will be so much more involved, know much more about how the school is run, have more of a voice in decision making, get to know the kids and teachers much better- all things that people with kids in regular preschools often COMPLAIN about not being able to do.
But how much fun this is for you depends on SO many different factors.
Are you looking to make whole-family friendships, or just to send your kid to school? (there is nothing wrong with the latter, either. You're a grown up and can make your own friends however you want!)
Do you want to be a room mom that often? Do you like and relate to the other families there? If you don't share similar viewpoints and philosophies - at least in regards to education- it will be a lot harder for you to work with this group. Or you may make really close friendships here that will be for adults and kids, your whole family, for YEARS- if you 'click' with the group. But you have to judge for yourself if you think that's likely to happen.
How do you really feel about the quality of the education your son will get (it IS pre-school after all, and not pre-med) and the general atmosphere?
This sounds like a pretty standard amount of involvement and work for a good co-op. Everyone has to pull their weight for it to work out. Some people do this and end up LOVING it- and some people it is just too much of a time commitment. You have another child whose school is also going to be asking for your time and money, don't forget. It can be hard to balance that and no shame to you if you can't or don't want to.
But- you may find another 'family' here at this school, with other parents to help and support you while your husband is gone. I have a good friend who has been part of a MAJOR babysitting co-op for years and although she certainly has to do her share of the work and scheduling and babysitting- she has 3 kids of her own and doesn't know how she would have managed without it, since she also works full time.
Maybe you should just try it for a semester and see if it works out for you. You may love it. If you don't. I would just explain that with commitments at your daughter's school as well, you just cannot give the time they need and withdraw with no bad feelings on either side.
Good luck!