BD's program gives you a 25% mileage bonus not only for flying BMI, but also any *A carrier. That is something you won't get with M&M.
On top of that your miles go a hell of a lot further with BMI as you can book rewards using miles+cash. Return Europe-Asia-Europe BC award on a *A carrier is 120k miles, which is pretty much standard practice, but you can opt to use 1/2 the miles and pay 271 GBP. So, 120k miles can get you a round-trip BC ticket... or it can get you two return BC tickets for just 542 GBP more.
