If price is the main factor then choose whichever is cheapest, I had a great trip with Bmi to LHR
in 2003 on their A321/320, got a cockpit visit and crew were fantastic which made the trip even more memorable. Both bmi and BA
have similar service short haul, do Bmi do the hot breakfast on morning flights like BA
do? Thats the only thing I can think that separates the two apart from the different a/c types and timings. Whichever you chose i'm sure you won't go wrong. But if you haven't tried British Airways, i'd go with them!