Why not join BMI Diamond Club instead of M&M?

The thresholds for status are very low - 16k for Silver and 38k for Gold. Compare this to M&M, which needs 100k for Gold (135k for German residents).

BMI, unlike M&M, also has 'cash and miles' awards which let you use half the number of miles normally needed in return for a small cash payment. For eg, a flight to the US with BMI is 37,500 miles plus GBP 255 as co-pay (plus tax), or 75,000 for all-miles. With M&M, it would be 90,000 miles.

Reward availability is the same as you'd get with M&M. BMI miles also do not expire as far as I know.