Not that it's any of my business, but I find it strange you are deciding this on your own - doesn't your husband have an opinion?
I'm in a similar situation, 2 boys and thinking about whether we want a third child. My husband always wanted 2 children and I always wanted 3, but sometimes now we each have the opposite preference. He's concerned about the extra cost of having 3 and I'm concerned about the extra time (and having less time for each child individually). For me, the idea of having 3 kids is more of a gut feeling - feeling like that's what would make our family complete. We would both like to have a girl, but I almost think my husband wants a girl more than I do :-). If we decide to have a 3rd, it's definitely not going to be so that we could potentially have a daughter, and I don't think that's a good reason to have another baby. For myself, I'm working on trying to separate my desires for experiencing being pregnant and giving birth one more time from my desires to have another child - if that makes any sense. OK, I'm pretty much rambling here, but that's what came to mind. Good luck with your decision!