The Naim DAC is quite good IMO. It has a few features the BDA-1 doesn't have, which drive up the price - most notably iPod digital decoding and ability to use a seperate PSU. The Naim DAC also has DINs, which are a Naim thing, as I'm sure you know the difference between them and RCAs.
The difference between them is the stereotypical difference between Naim and Bryston - wall of sound vs 3D imaging, Bryston's precision vs Naim's aggression (in a good way), etc.
I'm a huge Naim fan. I like the BDA-1 more. Naim guys tout PRaT, but I'm sure you're hearing just as much of it in the BDA-1 as you did in a Naim source. Sonically the BDA-1 does everything the Naim DAC does, and adds a dose of hifi attributes too.
If you're in Europe, the price difference will be different. Here in NA, the BDA-1 is about $1k cheaper. I think it's a no brainer, but that's just me. Synergy is an important factor though. I don't foresee any synergy issues with the BDA-1, but I haven't heard it with a Naim system.
The Naim DAC gets bashed a bit on the Naim forum. Can't see why. I heard it with the top level non-500 series Naim seperates and S600 Ovators, and it was one of the best systems I've ever heard. It's no slouch.
