So only thing going for Player Pro is better UI and more efficient at grabbing album art? I'll stick to Power Amp since I don't care about album art and it does everything I need of it. I also don't find the UI horrendously ugly since all I use is the (bare) widget to control my songs. The bolded statement is enough of a reason for me to pick Power Amp.
Will give Player Pro a go though

Question, does it sort by folders on the SD card since that's how I organize my playlists.
I agree that sound quality beats out pretty much any other feature.
However, I'd discourage making any blanket assumptions on sound quality as it differs from device to device.
On my old Defy, PowerAMP sounded great with or without Pre and EQ. On my Galaxy S2, I had to turn off PowerAMP's Pre, as it was introducing distortion, even at low volume. ( I tested this with in-ear headphones I had lying around from Samsung, Sony and Denon, as well with a pair of JBL Monitors and KRK V4 studio monitors.)
This could be an issue with Android 2.3 or with some internal firmware thing on the Samsung Galaxy, I have no idea...
As for the interfaces: I find Player Pro to be much more polished, intuitive and convenient.
In PowerAMP, I can't switch from the Library screen to the Player screen and back, without the program having lost my position in the Library.
PowerAMP also won't let me block any folders from scanning except 'Media' which is kind of pointless, cause that's where the music folder resides under.
Further, while PowerAMP is successfully reading and displaying tags and embedded cover art from my M4A files, it still lumped all M4A files together under "Unknown" as if it couldn't read those tags. Thats downright bizarre.
Shuffle keeps resetting itself for various reasons, like plugging in USB and other system events that activate a rescan...
Finally, I find its interface a bit convoluted and also inconsistent. This is just a minor bother to me, its things like the shuffle button being too close to the forward button, and if you hit shuffle by accident, it instantly switches songs, instead of waiting to the end of the current song, if just to give you a chance to deactivate shuffle, so you can keep listening to what you wanted without having to go hunt and search for it all over again.
Anyway, I can't say nearly as much about Player Pro, as I haven't purchased it yet, just running the free version atm.
As far as that goes, their interface - I don't mean the graphics with that, but the flow of interaction - is very polished, it hasn't made me go "what the hell?" once yet.
The free version at least seems to have a lower feature count than PowerAMP, but what it does have worked without weird surprises so far.
One last remark towards all Android Market Developers:
I understand the desire to moderate your forums, to avoid flames and the kind of negativity that would discourage newcomers from buying your goods.
That said, when I get to a forum that seems overly locked down, I do start to wonder what they are afraid of, which fosters thoughts of taking my business elsewhere. I made 2 posts on PowerAMP, which were plain, matter-of-fact questions in regards to issues I had and couldn't find mentioned in their FAQ. Two days later, they are still not in the forum, which of course prevents me from getting any answers from fellow users.
So two things:
1) if you're gonna choose to allow or disallow posts to your forum, do it quick and notify your customers of your decisions.
2) if you block people from posting their questions in your forum, its probably going to end up in your ratings in the Android market, where you can't even respond.
-----> if anyone knows what the deal with the M4A file tags is, I'd love to know, cause right now, 80% of my music in PowerAMP is sitting in a big lump of "Unknown".