I'd think with the new vocal harmonizer many curent S910 users would want to up-grade.
I have yet to see a VH2 demo that is worthy of "professional live gig use" using the new so called VH2
Since you already use a very neat external vocal harmonizer (that you say you really like a lot), and
most of your act is
singing, you'd probably be best sticking with the S910.
The Tyros4's VH-2 is a
far better harmonizer, totally "professional" in every way, but the only way
you will really know, is to
try the one on the Yamaha Tyros4yourself....you must have access to, or have at least tried, a Tyros4 have you, Donny?
It's been out for quite awhile.
Ian