Hi!
If you have a BDA-2 and intend to use BDP, then you will find there is no difference between USB and SPDIF connection. The BDP-2 and original version of BDP-1 both have identical audio processing so they sound identical. The BDP-1USB is identical to BDP-1 in every way apart from the fact it has no conventional digital outputs and uses USB as it's output instead. So if audio quality you are after, with BDP-1USB you can do as good as you can do with the other two players because, like I said, there is no difference between USB and SPDIF.
Howver, I must say this has little to do with the type of interface and a lot more to do with the BDP itself. Use your BDA-2 with a conventional computer and then use it with the BDP and the difference will be apparent and it will sound clearer.
I would say this is because BDP has noiseless power supply so both interfaces, USB and SPDIF, work to the fullest potential. Considering measurements of both interfaces are virtually identical, they do in fact sound identical and any difference would be a matter for academic debate.
I use the BDP-1 and BDA-2 with an AES/EBU cable. I just like to use the hardware processor of the BDP-1 rather than it's software decoders but I will admit I hear no difference between them.
Cheers!
Antun