Andy,
LAS - #4-sized, straight sides so useable for shooting
BUS - #4.5-sized, no good for shooting, compatible irons with the BUPP and BUT (as Neil rightly says)
For a dedicated smoother, the BUS is infill performance at a pittance (don't tell Rob or he might put the price up :-$ ). For more flexibility, the LAS makes a rather nice shooting plane, and is preferable if you prefer a smaller/lighter plane anyway. Either way, I can't see you being unhappy, and if you were then Martin/Rob would sort you out in a brace of shakes anyway. A win-win situation. :wink:
Cheers, Alf