Willy,
my advice to you is to buy whichever suits your needs at this moment. There is no such thing as "future-proofing" as it is always inevitably expensive.
I'll give you a few examples.
1) BDP-2 can be upgraded to BDP-3 for 1.500 dollars. The motherboard in qustion cannot possibly cost more than 150 dollars and this is if you order one or two, not a hundred of them.
2) BDA-2 was an upgrade to the BDA-1 with a promise of DSD playback through future firmware update. That firmware never came because of limitations in the sample-rate converter within the DAC.
So you are either looking at a very expensive upgrade or no upgrade at all.
In any case, I have a feeling Bryston will move in a different direction withing the next few years and start making integrated DAC/preamp solutions. This is what others are doing and Bryston will have to follow so whichever you buy, it might be technologically obsolete within the next few years.
My advice would then be this: if you need DSD, buy the BDA-3. If you don't, buy the BDA-2. The differences are more subtle than you think.
