As far as I know (and I'm not certain here) Prestige is thinner gauge than Infinity. My wife's Soma Mixte also has Prestige main tubes and Infinity fork/stays. I think that's pretty typical across Soma's line of bikes.

The smoothie is praised amongst its owners. Seems to be a great frame for the money (new, at $400+), but not sure of the value if used, even considering the seller is throwing in a CK headset and a brakeset. If the frame is like new, then maybe it's a good deal.