Functionally, there's very little difference; it is all in the voices, and the A5000 doesn't have a vocoder/vocal harmony. In fact, since they both have 1Gb sample ram, but the A5000 doesn't come with any pre-installed, you technically have more space to work with (unless you're willing to remove the pre-installed Latin, Oriental, Europe packs. The African pack takes up very little space)
And as the A5000 has revo drums, while the sx700 doesn't, technically you don't need to run all future revo styles through the revo cleaner (out of the standard Revo compatible kits on the sx900, the A5000 has the Pop Drum kit, which means you *could* replace all the Revo drums on new styles with that one and they should still work)
But being based on the sx600 chip, there are no sampled solo strings (violin, cello), and no ensemble string samples that are lower than cello (no contrabass, like what's included in real strings or Kino Strings), no sampled Ooh/Aah choirs (like the sx700/s970/sx900) have.
Actually, functionally the A5000 does more (you can create and edit styles with multiple time signatures, and change scale tuning on the fly without using a registration) so if you don't care about strings & choirs, nor VH, really need incredibly detailed Oud and Kanoon and Middle Eastern drums (not available in YEM packs), then yeah I would recommend the A5000.
Note that while the A5000 does start with more of a blank slate, I guarantee that if you want the Middle Eastern sound of the A5000, you're going to want to install the Middle Eastern Oriental pack (at least) to make up for the very weak Super Articulated Watariat strings built in; ironically the old PSRA2000/A3000 Watariats sounded much thicker (and luckily are available as downloads)
Mark