I have had a BDP-1, BDP-2 with the ESI Jusli@ bord and now a BDP-2 with the BUC (IAD) bord. If your BDP-1 si working fine and is powerful enough for your needs in terms of size of your music library, then I really do not recommend an upgrade. Truly.
However, if your music library is too large for your BDP-1, then you might want to consider an upgrade.
If we are talking about audio quality alone, for all intents and purposes, there is no difference. I have had both BDP-1 and BDP-2 stacked on top of one another and spent several days intensively listening to music I am intimately familiar with and I could not positively differentiate between the two players. When I say "intimately familiar" with the music I listened to, I am thinking about studio albums our orchestra made two decades ago. I was in the live room playing, I was in the studio room during the mixinig stage and I Know each and every musician by name, type and maker of instrument as well as the model of microphone used to capture the sound. I Know each and every little quirk and mistake done during the original performance. It doesn't get closer to the original performance than that. Don't think of this as bragging, I am merely stating facts and the facts tell me upgrading to a more recent BDP model is a waste of Money - if we are talking about audio quality only.
I wish I could have kept my BDP-1 as it worked perfectly otherwise. BDP-2/3 models produce way too much heat to leave them on all the time, the switch-mode PSU driving the display can produce a faint hihj-pitched tone coming from the machine, the internal battery has a limited lifetime and can prevent the machine from booting properly. They are more complex Machines, if more comprehensive, and this obviously means more things can go wrong.
In the end, do not forget BDP is a transport, nothing more. The DAC itself has a far greater impact on audio quality. In terms of jitter alone, the point where it becomes critical is the input receiver chip inside your DAC. No transport can compensate for the errors that might happen down the chain, inside the input receiver. Fortunately, most manufacturers have come to understand this so buying a high-performance chip almost certainly guarantees jitter-free operation. To be quite honest, some up-samplers make a far greater difference than the quality of the transport.
Following this logic, I think I can safely say there is no difference between a BUC-quipped BDP-2 and BDP-3 because if BDP-1 and BUC-equipped BDP-2 sound the same while having completely different audio boards at the same time, then what is there else to say.
Hope this helps!
P.S. The topic of digital audio transports intrigues me and I have several modified DVD players that measure essentially perfect so I couldn't let this topic pass.