I'm leaning towards the M80 because:
1) Other people who have expensive ukes use it, including Craig Chee, Sarah Maisel, Matt Dahlberg
2) Although the RB is probably more durable than the M80, that means it's thicker and maybe a little more noticeable. The website says the RB is 29x9x11.5 which is slightly over the 45 linear inch limit for carry-on luggage. The M80 seems to be thinner than the RB so maybe it gets under that limit. It also has a shoulder strap, so it should be pretty unobtrusive when slung over your shoulder.[/
I have a concert M80 and it is perfect for travel. I really like the sling strap and the protection is fantastic. You don't want anything piled on top of it, but for normal bumps, falls, tips... I find it way more protective then a regular hard case. I would say the advantage of the mono is the smaller dimensions and tighter fit. My concert covered bridge fits like a glove. Not too tight, but no play. That said, I have not used a reunion blues, and believe others who say they offer more protection. However, I'm very happy with my mono for this uke.
Not that it matters, but what uke is this for?