I'm trying to come up with examples of when b2b releases by an artist are hotly contested as to which is better. The ones I've got so far (and my personal preference comes first each time):

Weezer vs Pinkerton
Alligator vs Boxer
Crooked Rain, Crooked Rain vs Slanted & Enchanted (haven't given much time to S&E yet)
Revolver vs Sgt Peppers
Doolittle vs Surfer Rosa
Kid A vs OK Computer (can't decide on this one actually)

I know there are plenty of others.