Can't answer your first question but I have a 521dab and love it, updated it for the first time since 2012 when it was bought and it feels a lot faster although that may be placebo
Only thing that gets me is the lack of psychical buttons, so I find myself takin my eyes of the road for things like pausing and changing song, which just makes me wanna buy a multi function wheel more
What's the difference between the two units? Apart from dab of course?
