I'm going to go against the grain here and put in a vote for the yellow gold. The reason is that yellow gold has more staying power and is less trendy than the champagne gold, which is likely to appear dated in a few years.
I guess it really depends on which is a higher priority for you: having a very current look or having style longevity. If you want something that looks more "now" and don't mind if it looks dated in a few years, because being very current is more important, than go with the champagne gold. Just keep in mind that if it does go out of style or looks dated, then it might not sell as well or get as high a price (if you sold it). If on the other hand, you prioritize a piece that will look good for many years (even if it isn't the "it" color), then the yellow gold probably has more staying power.
Of course, no one can see the future either, and for all I know champagne gold could be the new "classic" for decades to come. Experience however, shows me that simple silver tone (in the form of silver, white gold, or platinum) or gold, are the classics and the others come and go.
