Well, ...As you mentioned, it really depends. The SS was top as far as horsepower, ...the Z was top as being an SCCA racer. Take a look at the 4th gen cars, ...a Z28 was 'upfitted' to an SS (hence SS might be considered 'top' there).
I know you're looking for an answer as to which would be considered 'top', ...but it really depends on who you ask, ...and what is 'top' to them. These days, 'hp' is touted quite a bit, ...so I'd guess "SS" would come out on top in an opinion poll.
If Chevrolet does keep both labels for the 5th gen, ...I think (in holding the heritage of the names) the right choice would be for the SS to be on top with regard to horsepower, ...and the Z to have the handling suspension upgrades available for track use. I'm sure there will be 'packages' available where there will be a lot of overlap (ie. hp, suspension, both, etc.) but that's how it is these days.