To answer your specific question, you should look at the MSB, LSB, and PC values which are listed for the voices of two different Yamaha models to see if they're the "same" voices. For the "panel voices" it's possible that the MSB or LSB will be different even if the voices have the same names, but some of them might have the same MSB, LSB, and PC values.
As Biggles pointed out, even if those values are the same, the voices on a PSR-S model will most likely sound "richer and fuller" than the "same" voices on a PSR-E model. That's because the PSR-E models can use only one element per voice whereas the PSR-S models can use multiple elements per voice. An "element" is basically a single sound sample, so being able to use two or more elements per voice means that two or more sound samples can be layered together to create a single voice, resulting in a "richer and fuller" sound than if only a single sound sample were used.
But as suggested, it's best to compare models from different manufacturers before you make up your mind what to get. You can take other people's opinions and suggestions into consideration-- which can be immensely valuable if someone has extensive experience with two or more different models or brands and can point out the pros and cons of each-- but you should always try to "test drive" any models you're considering so you can form your own opinion which one feels best, sounds best, and operates best for your particular needs.