Realistically, I'd say that the iPod Touch 4th gen should be the premium choice if phone service is not a priority or necessity at all.
Both have about the same tech specs, with the 4th gen having a slightly faster CPU, and slightly faster RAM (which helps with graphics), but in day to day use, both devices are about equal in terms of performance.
RAM amount is equal on both, but with the 3GS having a quarter the pixel count, it can multitask better depending on the app, so that's quite a factor to take into consideration.