Jessica vs Krystal?
Personally, I think that with makeup on and in 写真 Jessica is prettier. Krystal IMO looks prettier than Sica when they're not wearing makeup and probably in person. But Sica's gorgeous with makeup on and in 写真 and videos. Krystal seems a little plainer to me (I know people think she looks like Yoona, but I think Yoona's prettier :P)
Who's a better singer?
I'd have to go with Jessica on this one. She's had ore training, and therefore has もっと見る technique.
Who's a better dancer?
Jessica had もっと見る potential, but Krystal practiced もっと見る so she's now a better dancer.
Who has a better personality?
They both have positives and negatives for me. Jessica doesn't pretend to be happy and cute all the time, and I like that, but she can seem mean. Krystal seems friendlier and もっと見る approachable but she can be kind of annoying.
In appearance, I'd say Sica's cuter, but Krystal acts cuter.