No sonic advantage. In fact, in some systems it may introduce noise from the HD or the HD PS into the Touch.
When you attach an external HD to the Touch, it activates the internal version of SB server, Tiny SB. Your Touch then runs off of that and not off of the SC on your PC server. Tiny SB is much the same as the big version, but you lose some secondary functions, like the plugins. Some users report good results in sonics and usability this way, others don't. Apparently some external HDs work well with the Touch, and some don't. You just have to try your drive and see.