ADL does not have a detachable magazine or hinged floorplate while the BDL does. Also BDl, depending on model, has black pistol grip cap and foreend tip, engraving on the reciever and floorplate and cut skipline checkering. I think some of the BDLs have nicer bluing, but I am not sure what the difference is.
The floorplate/magazine is the big difference. The ADL is the "lower end" model, but magazine is the only functional difference that I know of.
There are also synthetic and laminate stocked ADLs, but I don't think I have seen any of those. ADLs are traditionally available in a smaller collection of calibers (but still a very good selection), .270, .280, .30-06, .308 etc. and are probably not available in some of the more unusual cartridges. If you want a .17 Rem or .338 Win Mag you're probably looking at a BDL.
The wood stocked ADLs probably don't have as nice looking wood on them as the BDLs.
I'm a real cheapskate but I always spend the extra for the BDL, that magazine in the ADL is a PIA! Every time you go in the house or a car when hunting you have to rack the rounds in and out-it's a much bigger pain than it sounds. I also prefer the detachable mags.
I have seen bolt guns where the catch on the hinged floor plate wore out. My buddy got a great deal on a Ruger .308 that seemed like it was in good condition. When he fired it the floorplate would open each time and dump the remaining rounds on the ground.