As a fan of both, I would have tickets to see either of them as Dolly. My seats would likely be closer to the stage for Patti.
I do think the intersection of Bette's persona with that of Dolly just seems like a perfect fit. So, in her case, it is more about making sure it becomes a fully realized role and not Bette as The Divine Miss M as Sophia Tucker as Dolly. (Which, by the way, I would totally adore). That would please everyone, but for me to think I saw something transformational and historic, I need to see Bette use all of those skills to build something newish.
Whereas Patti doesn't have that arsenal (or baggage, depending on your perspective) of characters and would just get to see how she could create and interpret the role a bit more from scratch. So, we wouldn't necessarily know her choices as well in advance.
That said, at 67, one would hope that when Bette steps down from the role, this might be Patti's only chance to step in and do it, so... *fingerscrossed* Best of both worlds!
With a role like this, and the caliber of performer who will do it, the only expectation will be to get their unique take on it. I'm all for Hello Dolly turning into the Hedwig for every diva on Broadway. Get them all up there until we run out!