Aniston has an ass now?
Call me crazy, but I’ll bet there are thousands of women out there who would love to have an ass as “big” as JLO’s.
It’s just like my dad always says, “They’re all good, son. Just some are better than others.”
With that in mind, I’d say Aniston’s slightly edges out JLo’s.
What are we talking about exactly? Look? Feel? Taste?
I would say look, Homer J., though if you have any experience in the other areas and/or are willing to acquire it in a legal manner we will be glad to hear. 
Like several posters before me I fail to see how Aniston even qualifies for such a comparison. Her butt is no more (nor less) eye-catching than that of 90% of the women you’ll cross paths with on the street any given day. What’s next, praising someone’s elbows/forehead/insert-bodypart-of-your-choice-here just because overall they’re considered “beautiful people”? All eyes on the butt here, Aniston isn’t even a contender.
Aniston doesn’t even have a butt. If I had only those two to pick from, give me Lopez any day.
From a strictly aesthetic viewpoint, I would say Aniston. From a logistical perspective, I would say Lopez, because if I’m seeing her butt that means she’s walking away from me, and any activity that takes her further away from me is a good thing.