I just don't think Ada's that great. She's a spy...but she's not very good at it; her emotions get in the way and it's very hard for her to keep her personal life away from her professional. Jill's always been very calm, cool and collected in dire situations and is able to think on her feet because of it; she's also very professional. (Which is probably part of the reason JillxChris hasn't been outright confirmed in the series. They're very professional despite their feelings unlike Leon and Ada.) I think Ada's more emotional side hinders her character in some ways that being cool in stressful situations strengthens Jill's character.
Think about it this way, Claire's not as good at thinking on her feet as Jill is because her emotions are always on the surface and, even though Ada's not as emotional as Claire, she still lets those feelings get in the way of her job and it will hinder her where as Jill has the advantage. Put something they both care about at stake while they're fighting and Ada's a goner.
Jill also has more field experience if I'm not mistaken, not to mention the fact that she's older and a little wiser. She's been through more. Someone earlier mentioned that Jill survived two outbreaks and Ada barely survived one. It's true! Ada's just not very good at the whole survival thing whereas it seems to come natural for Jill. (Even Rebecca survived two with no problem.)
Jill was enhanced in RE5 but not in the scene I was referring to. The scene I spoke of was a flashback before Wesker captured her; she was still human then.
