Shorts and bibs are like shoes and seats. Very personal. If the Black Bibs work, why change? Maybe buy one pair of the nicer ones and see if it is worth the extra money. And now you know that shorts that can double as diapers don't work for you, simplifying the search.

Yes, too big can cause issues. For me, too big shorts means my personals not staying where they belong. That will get me long before I see a rash from a chamois fold. But again, what works for me probably isn't what works for you.