A few thoughts....

• Transporting a folding bike won't offer quite as big an advantage over full-sized as you might expect; I always found packing a folding bike to be a PITA. You will likely avoid fees though as long as the bike is under the weight limits.
• The Xootr has a very solid feel and design, and you have a bit more adjustment options.
• The Dahon has a much better fold and (afaik) a wider gearing range.
• Performance will probably be about the same.

Not sure how big a deal the proprietary parts thing is, since the higher-end Dahons tend to use more standard parts than the cheaper models.

I think you'll be fine with either one, as long as you don't expect miracles when flying with the folder.