If you go with the 'add-on' board, the same area under the board will be blocked - you'll have to remove two hex nuts to remove the add-on board to get access to what's underneath it.
if you go with the all-in-one combo board, you'll have to remove 4 hex nuts to get at whatever is under it.
Really not much of an argument to sway me.... price might be the only argument, but we don't know that yet.
Regardless of price, I'm going to be purchasing the all-in-one / combo board - in my opinion I get:
(a) a cleaner solution.
(b) newer/cleaner/better optos (hopefully same type that GLM uses on their trough opto boards)
and perhaps the best benefit --
(c) a spare 'original' trunk opto board - to preserve and use in the future if ever needed. Worth $40 bucks itself per Marco Spec.
-Brian