I would take the EX500 anyday over the GS500. I'm not saying that the GS is a bad bike, but it BOTHERS me that it is all show and little go.
I say and think that because it is a late 70s engine on a mid 80s frame with new looking plastics, but other than the plastics, it is a dinosaur performance and tech wise!
The EX is no state of the art technology by any means, but at least it is closer to a late 80s bike, with a DOHC 4 valve per cylinder engine and watercooled engine.
IMHO the watercooled engine is a critical factor, better driveability, more refined engine, less wear and if taken care properly, as reliable as an aircooling system.
Everybody knows they could do WAY BETTER with those little 500cc engines, why not repeat the history again? Those engines started their lifes as 4 cylinder engines that were basically cut in half, why not do it again?
Both bikes could be viewed as dinosaur-bikes, I don't think there is an older design being sold brand new (I'm talking about sportbikes, not cruisers, heck, not even commuters are that old). I think that apart from the EX and the GS the oldest one would be the ZZR and the engine on the Honda 599.
What bothers me more is that in Japan you can get a 250 4 cylinder bike that makes more power than the GS, and it is a Suzuki!!! I can only dream about what they could do if they really tried to with 500cc of displacement, but I guess we'll never know...