My two cents:
I've stayed at all of the Atlanta properties -- W Midtown is best for upgrades. I've never had a 'meaningful' upgrade at W Downtown though the property is newer. Also, the vibe at W Downtown is very young/hip so keep that in mind. The weekend pool scene is filled with the 'beautiful people'... You'll have to decide if that's the scene you're going for.
As far as 'The Peach', pop on over for lunch/dinner at the Sundial but don't waste any nights staying there -- it's old and not in the best shape.