The P880PRS gains on the JVC for its dual 16-band graph EQs. Pretty much no other HU on the market has that. The polarity thing, athough a feature, is as simple as switching the + and - wires around. Some people use it as a tuning tool. I see it more as a lazy fix for not paying attention to the + and - wires when hooking things together. It can be useful to some extent if you don't have TA. The internal amp is not a huge thing, no. To some folks who might not want an external amp, it might be important. Both units offer almost identical x-over adjustment. The JVC does have a SD card reader, and it's nice to see flash media support. SD cards are quite cheap too and more compact than USB. Some PCs like my Dell laptop even have a build in SD reader(although the JVC unit comes with a SD to USB thing). When buying my AR8500, I passed up the P860MP/P960MP for it. The P880PRS didn't exist at the time. Although the independent EQ and 16 bands is nice, you also have to decide if you will even make full use of it. I see the P880PRS and AR8500 as a more chocolate to vanila sort of argument. They both have
enough features to keep you happy, but each has their own taste. You just sort of pick the one that better fits you.
Why would I give up my own AR8500? Well I gave it up once for a Ural Concert CDD

cause I was stepping to a 4-way setup. Then I got a PPI DCX-730 to allow more HU freedom and to gain subwoofer TA and put the JVC back in cause, well, PPI + Ural is massive overkill and the JVC is overkill enough. The PPI doesn't like cold weather(sort of making it useless), so the Ural's going back in and the AR8500 along with the PPI goes bye bye again. The Ural unit is something that stomps both the JVC and Pioneer units feature wise. However, in a 2-way + sub configuration, I really wouldn't care which HU I'd have in my car as any of the half dozen or so affordable, 3-way active HUs now available would be "enough" to be happy with.
There are quite a few 2-way + sub active capable HUs on the market these days, enough to shake a stick wildly blindfolded and probably hit one. They all have enough processing power to do what you need it to do and keep you happy. However, it's all just a matter of taste as each one will offer slightly different feature sets, a different user interface that you may or may not like, and different looks that may or may not be attractive or match the car's interior. It's sort of a best fit thing.
I will make an additional note with the JVC units. They are tanks. You can buy any of their HUs throw it in your car and not worry about it wearing out or breaking pretty much for the life of the car. I've had to throw away two of my JVCs because they outlived my cars and their use in my progression towards higher end HUs and more features. One was an old tape deck of about 10 years and the other was one of their gaudy Kameleon decks that I had for about 5 years, talk about motorized and flashy lights to high heaven, lol. I have a friend with a very low end JVC CD player that he used for a good 3-4 years until he bought a new car that it can't fit in. They don't break. They don't wear out. About all they do is get dusty and old and generally out live the car or their usefulness. Hitting below the belt, JVCs have one bonus over Pioneer, no RCA hiss:laugh:, and I know that's a big stickler for a lot of people considering Pioneer units.