I also have both, but I also have had at least 4 of both over the years...Beautiful styling in the Big Ben, I kept the silver japanese version, and it sounds amazing, but as others have said, its no boomer in comparison to an M90. Not to say its quiet--its not.
But the M90 has the ability to be turned up to 8 or 9 and stay cranked without distorting if you are lucky enough to find a like new one...I like to classify my radios into outdoor(basketball court, I coach), and indoor(study/library--I read
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).
Big Ben is amazing in any study, super beep btw, 8 inches I would say. the M90 is longer, shallower and slightly shorter, easier to carry and it seems tougher, like it could take a knock a bit better although I'm not testing that theory!