The dedicated entrance for the bus drop off for First and Biz class is at the very end of Concourse B so you have to then walk the 1km to immigration if you are entering Dubai whereas the economy bus drops you off in the right place to get straight into immigration area. At least that is how it was a couple of months ago. Since then I have always insisted on getting on the economy bus. It may work out marginally better for transit pax in F and C though to get the F and C buses.