Traveling conundrum

I have a soon-to-be 5 year-old son with undiagnosed ADHD (his father and grandmother have it, all of his doctors agree he has it but won't issue a formal diagnosis until he's 5), and I have a 2 year-old daughter that does not appear to have it.

I come from a large family, the oldest of 7 children, none of whom appear to have ADD/ADHD nor have been diagnosed. As a result, there is not a lot of tolerance from them for the ADHD "antics" of my husband and son. As a result, sadly, I have to limit the exposure of the two groups to each other in order to foster "good feelings" between them, rather than fostering anger and resentment.

One of my brothers is moving with his wife and kids to Japan this summer, and I don't know how long it will be before I get to see them again. So, we're ALL gathering at my mom's house to bid them farewell. We're all spread out, clear across the country, so this is a big deal.

My problem is that in order to take my family of 4, the trip will cost around $3K, because the ADHD requires that we get a hotel room for the duration of our stay. (Just makes things, and routines, easier on me.) The high cost has dictated that I go, and take one of the kids, leaving my husband with our other child. But who do I take?

Do I take my daughter, who has become cute and fun, travels well, tolerates schedule changes well,  sleeps well, and won't remember this? Or do I take my son, who is funny and bright, doesn't travel well, doesn't tolerate schedule or environment changes well, doesn't sleep well, and will remember?

Even flying, we'll be traveling all day just to get there, crammed onto airplanes, awash in a sea of people, nothing to do. Pretty much the way we'll be at Mom's LOL. If any of you have some words of wisdom for me, I'd appreciate it. I know that no one else can make the decision for me (which would be nice, because then I wouldn't be responsible for the consequences, right?), and I know that in the end I need to choose what's best for my family. I'm just having a really hard time making a decision. Thanks.