Welcome to FT! (my 3rd or 4th in over a year)

Based in EU and flying *A will involve LH. For US flyers, AC or Asiana is good for lounge access in the US.
For your style, I think LH may be the best (or only option, as LX and BMI etc have been assimilated)
If you are SEN, you do get treated well and they give double miles for Biz.
You do realize that B is an economy fare and Biz class is C or J, I hope, as Biz class will be better treated and you can get status easily. LH gives double miles for Biz class and offers some reduced awards if you take a companion along.

I know very little about LH, except that I signed up my kids as my brother was a SEN and never again flew them in paid C, so their accounts are dormant. If you lurk in the LH forum, you may learn a lot more about them.