R700 appears to no longer be in the current lineup, replaced by R710.... I think this is a good move, because quite honestly, my previous advice for R700 was "basically never" over R500/R600. I've never noticed a performance or capacity advantage to R700, despite it was supposed to be a higher end AP.
Long story short, the R700 is the 7982 with 802.11ac capable radios but largely identical analog components. Shortly afterwards, the R600 and R500 were introduced with newer antenna designs, and IMO these are the best 802.11ac AP's on the market in terms of performance and value.
As far as choosing between the R500 and R600, I think it boils down to whether or not you have 3x3 clients or not. For capable 3x3 clients, I've observed around a 20% throughput improvement on R600 vs R500. You'll have to decide if that's worth the price premium.
Of course, the R710 is a behemoth both in capability and price, and I've noticed a noticeable performance/range improvement over the R600 and R500, but I think it's a big price to pay for it. If you are ever deciding between two R500's or 1 R710, I'd still vote for the former.