I know there can be a lot of second-guessing on these boards, but....

Weight isn't the prime consideration. Sure, you're flying, but 99% of the time you'll be riding. Maximize the utility while riding. If the 531 frame can fit 30 or 32mm tires, fenders, and still have clearance for mud or if a rim gets out of true, then use it. If 28mm tire and fender barely fit with no room for mud and the wheel must be absolutely true to not rub, then forget it.

The frame weight difference will not be huge enough to matter, people fly with full-suspension mountain bikes. You might have to pack lighter things around the bike, but otherwise I wouldn't worry about it. Maximize utility while riding.

Of course YMMV depending on tires you want to use and if raceblades rather than full fenders will work for you. Good luck.