It really makes no to very little difference. An unfaceted girdle can help the color appearance in lower grade stones (K, L, M, etc), but at this point we are really splitting hairs. Most experts think that a faceted girdle is a sign of attention paid by the cutter during the cutting process, but you'll fid several Superideals with natural, unpolished girdles. All in all, it is really a matter of personal taste and diamond availability at the time of the purchase.
i agree..i have a stone with an unfaceted girdle, and its an ideal cut stone. it has zero effect on the brilliance and perfomance of my stone...and no differnce in the way it was set. you do have to make sure that if it is unfaceted, that there is very little to no bearding though. that can be determined either by the vendor or by an appraiser.