Hi dmc1979,
it is not easy to give the "ultimate" tip here.
The S900/700 series was introduced in 2007, the S910/710 in 2009.
Basically I don't recommend to buy such an old model (except maybe from a Yamaha dealer who gives you a one year warranty). If a hardware defect should occur on such an old model, a repair is no longer economical in most cases.
If you can get an S910 (instead of an S710), this is the one to choose. The S910/710 series can read both Style formats (SFF1 and SFF2), the S900 only supports the "old" SFF1 format.
The S900 has 23 Super Articulation Voices, the S710 does NOT have any SA Voices. But the other Voices of the S900 (e.g. Mega, Live, Cool, Sweet Voices, Organ Flutes) are also in the S710.
If you need a Mic Input and Vocal Harmony, you have to choose an S900 or S910. The S710 does NOT have a Mic Input.
P.S.
If you want to work with Expansion Packs you need at least an S770 (better are S970, S775, S975). With a view to Expansion content, I do not recommend to choose an S750/950, because those models are not compatible with the current YEM (Yamaha Expansion Manager) technology.
Hope this helps!
Best regards,
Chris