There are probably more "classic" 500 series Citations out there in the world, since Cessna have been making them for 30 years. On the other hand, for the last 5+ years, shipments of 525 series Citations have far exceeded 500's, and with the new CJ3 that will further erode the 500 line. The 525 is therefore definitely the product of the future.
But, as Kopeloi says, fleet statistics are not particularly important if you've got a job on offer, because that would be in a particular aircraft. And, as has also been said before, if you DON'T have a job on offer, then speculatively doing a type rating is very high risk.