Wow! This is a tough situation. I can see both sides of the coin. As a mom to a toddler and another on the way, I can only imagine the struggles I will have to face when they're older.
Have you spoken with the boyfriend's parents about this yet? What is their take on the situation? I think all of you in one room would be tight and awkward. Remember, teenagers can be very self concious about their changing bodies, fluctuating hormones and other issues that may arise in sharing a room. (I teach 8th grade so I'm familiar with many young, teenage angst issues

).
I agree with the one poster who commented on the fact that your daughter may pout and be cranky - I would NOT give in to that behavior. If you decide NOT to bring the boyfriend, telling her with a few months notice would let it sink in and she will have to accept it. I certainly wouldn't give in to her to appease her or to make my own life easier.
I also thought that the one poster's comment about keeping them attached to your hip ...
Since when does keeping your teenager attached to your hip and away from outside influences develope "resiliency and self-sufficiency"? It doesn't
I don't think the point was to keep them attached at the hip, but to let them know that they are still CHILDREN and cannot make some of the more mature decisions they think they can.
As a teacher of middle schoolers it is interesting to see how they act. They look like grown-ups. Some look older than I do! But their minds are out of whack with crazy hormones driving their ever-changing brains. Just step back and observe. I took my 8th graders to Disney in February - they performed in the MK - and watching them on the bus, in the parks, at meals, boy! They are really all over the place mentally. On one hand, they think they're all grown up. On the other, they're looking for mommy or daddy to make everything alright.
I think ultimately, this decision is YOURS. Not yours and your daughter's - just yours and your husband's. Talk to the boyfriend's parents. Decide who will pay for what and if you do invite the boyfriend to go, sit down with him AND his parents and openly discuss your expectations. I just wouldn't give in to your daughter simply to keep her "happy" while you're on vacation. EVERYONE has to be comfortable in order for the trip to be a success.