Well, *all* the sounds from the s770 are on the sx900 (and in fact, all of the s970 sounds are there; all of the s970 sounds are on the sx700, even!)
Anyway, if you don't select a new voice, playing the 'old' voices, of course it will basically sound the same (but through much better speakers),
You have many more voices to choose from on the sx900, and the exclusive voices (ie not on the s770/970) are the highest quality voices on the sx900
Mark