all three have IDENTICAL architecture, so you gain nothing from getting another... Yours is the only one with actual sliders. And you should be able to interface your ax-60 with the s900 as is... you just need to find this idiosyncratic 13-pin connector... they can be found on the internet.
THe AX-60 was my first "professional" synth (has a Casio HT-700 before that) that I purchased in the late 80's. I got hold of a VX-90 in the early 90's and sold it quickly since it has pretty much the same sounds as the AX-60. I think the VX-90 added velocity repsonse which could modulate filter cut-off. I think you could even vary the detune amount of unison mode. Other than that, it brought nothing more to the audio pallette.
Since joining VSE early 2007, this forum has made me miss my AX-60. Especially since it was the ony one I have ever seen with blue buttons for both BANK & PROGRAM.