This is a tough one for me. Both of them are my favorites and both kick a** in RE5. But if I had to choose it would be Jill since she pretty much defeated Nemesis in RE3 and also stated in the Resident Evil Wikia which I think is true since you can tell right off the bat "Jill is generally considered by fans as the most competent female protagonist of the series. She is more calm and collected than the more emotional Claire, and is a more mature and able combatant than Rebecca. Jill also has more biohazard containment experience, as well as better biohazard understanding/knowledge than Sheva Alomar. She also single-handedly defeated the Nemesis, one of Umbrella's deadliest bio-weapons to date."
NOTE: This is just my opinion.