The place Mom likes costs 6500/month, and she can pay for 4-5 years there. Given her siblings ages, she very well may live more than 5 years. She has middle stage dementia. Would it make any sense to go to the nice place, and downgrade later when she's less aware? Would later stage care be more expensive? She's just transitioning from living at home to away, and that's disruptive. Would move be preferable to two?
However, the other side of that is that she may need a nursing home in her later stages of dementia, unless this AL has a specific dementia unit and she has no other major health problems. My point being that she could, eventually, have to move anyway.
Yes, no matter where she lives, later stages will mean more money.
None of us can see the future. I think I'd make the decision on how much difference there is between the two. If the not so nice AL is really, well, not very nice, then I'd take a chance with the better one. If there isn't a lot of difference, I'd take the lower priced one because that could mean she wouldn't have to move again.
Whatever decision you make, don't blame yourself if down the road you wish that you'd done it differently. That's one of the hazards of caregiving. We have to do the best with the information at hand.
Good luck with the move and please keep in touch,
Carol