"theoretically", the R500s should be faster, mainly because they have less spokes, but if they're not as stiff as CXP33s they may then feel slower. Either way, I doubt you'd notice any difference.
I'd be more concerned about durability. This is obviously anecdotal, but I know a few guys who had problems with the R500s continually going out of true, and spokes breaking. These guys have been put off Shimano wheels for ever!
yeah, basically, but I'd add "less reliable" for the Shimanos. Also, advertised wheel weights are dubious at best, and often don't inlcude skewers, so the Shimanos would be more like 2000g with skewers. With straight guage DT Champion spokes, the Mavics would be about 2100g with skewers. Either way, I'd be amazed if the Shimanos were more than 200g lighter, which is nothing
Dawes Kalahari, Puch Prima Super Sport, Graham Weigh 853
The rim weights are probably the most important weights since they are at the outside of the circle. Cycling plus in the UK deconstructs the wheels and quotes rim weights separately. In my opinion, factory wheels should show the weights of the components. I expect that the rim weights of the two you compared are similar. You could easily drop the weight of a Mavic cxp-33 wheelset by choosing lighter hubs. I'm sure Ultegra would be a great choice.
Shimano Rxxx wheelsets are total crap. They are not even in the same ballpark as a decent set of handbuilts sporting Ultegra hubs.
The Shimano wheels have a low spoke count, but are actually very heavy. I have a set of R550s that weigh more than a set of old Mavic Open 4 CD w/ Ultegra hubs both being 32 spoke 3X.
Go for the Mavic w/Ultegra in both front and rear. Assuming you are fairly lighter weight (sub 200) then go for the lowest spoke count configuration you can get in the front (assuming around 28). Go 32 3X in the rear. Get some DB spokes in like a 14g-15g-14g, some brass nipples and call it a day.
In 20 years (assuming you haven't detonated the rim) you'll find they keep working their way onto your current bikes....still...