A few comments:
The sound of MQA is hotly debated. Some like it, some don’t, some think it’s sometimes better, sometimes worse. Personally, I wouldn’t make a purchasing decision of HW based on whether it does MQA or not. Choose HW for all the other reasons, and if it also does MQA, then that’s a bonus. If your purchasing decision is only between the two Myteks, it doesn’t matter, because they are both MQA capable.
IMO, that’s especially true with software like Roon that does the “first unfold” of MQA. If there’s a benefit to MQA, you are already getting most of it there. The so-called “second unfold” is really just a form of upsampling with MQA provided filters, and if you upsample those same tracks in Roon or in a DAC chip (even without MQA) you are doing essentially the same thing.
The BB and the DAC + are essentially the same in this context except one has a streamer built in and the other doesn’t. With the DAC+ you’d probably need some kind of server and then use USB. You’d need to add a streamer if you decide you want to use ethernet with it. With the BB you can start out just streaming and then add a server. They should essentially sound the same: the pre/input/volume section is the same, the DAC is the same, and the output section is the same.
Lots of people will tell you USB or ethernet is better, but it’s all BS, because they aren’t hearing your setup and don’t have your ears. One may be better in one household, the other in another household.
There is no definitive answer.
Is steaming more important to you or having your collection ripped on a local server? Will you be happier now with streaming and can forgo a server in the meantime? Or the opposite?
If I was you, I’d also think one step ahead: where do I want to be with my setup not just with this immediate purchase, but with my next purchase? What’s most convenient and will let me most enjoy listening? For some people ethernet/home network streaming adds more options and convenience. Others prefer the direct USB connection from a server. Some like having the streamer and DAC in one box for simplicity, convenience, less cables, and one less box to work with.
In ultimate price terms the two devices could be essentially a wash for you, b/c if long term you want both a local library and ethernet/home network streaming they both involve buying a streamer in some form and a server in some form. You need to decide if you could be happy with “just” a server connected by USB to the DAC, and listen to both your local library and internet streaming that way.
Don’t underestimate convenience and having the all the options you want in your setup. That will probably be more important to you in the long run than a possible-maybe upgrade in SQ that you won’t notice if you don’t directly A/B two methods of playback (and maybe not even then).