Why not do both? I think 500px has its advantages and its disadvantages. I think it's a higher quality, more upscale place where people are serious about their work. On the other hand, the potential viewers is less, and the uploads are limited unless you pay. I say maintain both and cultivate your best work over there. Give it some time and then decide. Please let me know if you do create one so I can follow you
A lot of limitations with 500px if you don't have a paid account.. but the algorithms in which a photo can get popular is way better as opposed to dA's flawed system. Plus, it focuses solely on photography with a better community and overall content. I'll be keeping both though. Definitely not a place to receive any form of criticism, but then again, i don't see many sites that offer good community support for that.