I am both LH SEN and BMI *S.

When making a comparison I would list the following points:

BMI:
+ Good burn ratio
+ Ability to book o/w awards
- Horrible Indian Call Center
- 5 business days advance booking
- No through fares to continental Europe

LH:
+ Better earnings on LH's cheaper fares
+ Excellent CC in Kassel (always call the German line)
+ Lounge access when making *S (35k miles)
- Worse burn ratio

2 r/t to Europe a year plus some domestic flying should bring you close to LH FTL (*S at 35k) which gives you Lounge access (in several LH US stations) and slightly higher service levels.

In addition to that, miles do not expire anymore.

So I would just stick with LH