For bibs: You put them on and they stay put, no waistband issues or constriction.
For shorts: You don't need to get undressed to poo (which you'll probably have to do at least once).
To echo what was said earlier about comfort: Skin/fabric interface will make or break your tour, which is where cycling shorts would excel, padding or not. I had sore spots under my sit bones after my first tour, but I was really green and rode 100 miles round trip. That was with padded knickers (not bibs). If I was dealing with cotton underwear, seams, and chafing, I could have done a whole lot worse.
Ride and ride and ride some more to get a feel for what you need. Short rides, long rides, there's just no substitute for listening to your body (when you're not miles from home) so you can make the tweaks you need.