I don't know much about the 31, but some time ago, Pioneer had rather poor FW support, and hopefully it might be much better overall (and it might hopefully be better overall for all of the companies who weren't so good).
The 23 cannot bitstream DTSMA, but can send it decoded. Not a biggie, but if you want to see DTSMA light up on the receiver, look elsewhere. It cannot stream Netflix, while the 31 looks like it can.
If you just want a basic good player, I like the Pana 65 for under $120. If you like having the most streaming options available beyond Netflix, look towards LG.
If you want to spend more for better upconversion of DVDs, I'd get the Oppo 80 for $289. However, I think differences in upconversion are subtle at best, and even more so when comparing to native BD.