Solid Hardwood, Engineered and Laminate Flooring - Leveling sagging subfloor

My 2nd floor subfloor slopes gradually from the outside walls and loadbearing walls toward the center of the rooms. The deflection is no more than 1-1/2 inches and there are no noticeable high or low spots. The subfloor is 3" pine tongue-and-groove in good condition, late 1920s vintage like the rest of the house.

Now I want to put a finished floor on top and I've had some estimates. I got one contractor saying "Don't bother leveling the subfloor, just use nailed-down 3/4" wood floor or nailed-down 5/16" engineered wood floor."

Another said "You have to level it regardless, and then you can use a floating floor or nail or glue it down."

Yet another says "Since the slope is gradual and you can live with it, a floating floor won't bubble."

So is it six of one or a half-dozen of the other? The slope is not readily apparent to us day-to-day, and obviously the less floor prep the easier and cheaper.

The question is... What are your expectations?

Prefinished or site finished?

Come refinishing time, if prefinished, may be a concern if it has a good drop to it.

The question is... What are your expectations?

Prefinished or site finished?

Come refinishing time, if prefinished, may be a concern if it has a good drop to it.
Expectations:
1. Floor will not move, bow, or bubble underfoot except seasonal expansion & contraction.
2. Floor will look good, of course.
3. Floor will not wear prematurely.

We would prefer prefinished because, though it can't be refinished as much, it seems cheaper and does not make the mess of site-finished.