I don't have Genos, but I did have a chance to try it in store -just so you know where I'm coming from with my opinion.
Yes, Genos has some voices that just don't exist on SX700/900.. and yes, they're good. But that doesn't answer the question how "important" they are. Because the answer depends on music you play. To understand what I mean, let me ask: if someone would ask you how important are MegaVoices (on SX900) to you, what would you say? Exactly, depends on music. Speaking for me, I don't use MegaVoices much in my styles -simply because they don't sound right to me for music I play. But there are songs that benefit if I use MegaVoice in style. That is, I won't use MegaVoices all the time just because I have them -I will use them when I need them. MegaVoices aren't "better" than normal voices, they're just different.
And yes again: Genos has overall "better" sound. But again, how much of a difference that is, depends on you (your ears). How to say, there is difference, but not "huge" -still, it can be big enough for professional music producers. For home and semi-pro use, difference is not really relevant. In your case, it will make bigger difference if you get good external speakers for your SX900.
That's my personal (very subjective) opinion and I expect it to be different than by Genos owners.
Bogdan