Have run both BA and regular marathons in 50x559 size on my gravel road mtn bike. the BA's profile is a touch wider and floats slightly more, especially at lower inflation pressure - in 40s lb. the marathon is more effective on gravel & dirt surfaces. went with 47 mm marathons on a crosscheck for country roads, it was nice for commuting also.
ps- if I had the room- might look at conti contact or comfort contact in 54-622,
one of those has sort of a smooth center strip and edge tread - these could
be a good balance of smooth running on pavement, traction in gravel and
float. during the past couple of years, have had two sets of conti tires-
they seem to run on the narrow end of their stated size.