Jessica vs Krystal?
Personally, I think that with makeup on and in photos 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 photos 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 more technique.
Who's a better dancer?
Jessica had more potential, but Krystal practiced more 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 more approachable but she can be kind of annoying.
In appearance, I'd say Sica's cuter, but Krystal acts cuter.