I would go with the BBK. Yes, it will depreciate, but neither choice is a good _investment_ IMO. I will get more actual enjoyment from the BBKs (and the M as a whole), whereas the Panerai will do the same thing my $100 one will, with an acceptable margin of error.
As for the discussion of if the BMW is a luxury item like the Panerai, it is not a good comparison. The difference in function from an Accord to an M3 is huge. From a Panerai to a Timex, not a large functional difference. The M is a luxury item, but you are also getting function for the $ you are paying. A nice timepiece on the other hand, is much more about appearance than function.
I agree that it doesn't matter what I or anyone else thinks other than the OP. I don't ever see buying a timepiece for myself worth more than a few hundred dollars, but I don't hang with a crowd that expects me to either. In fact, if many of them knew that watch was worth more than a few hundred, I would be surprised.
On the other hand, owning the M has nothing to do with what others think for me. I bought it because I like to enjoy what I drive. If it was ugly and neon green, I would enjoy it just the same.
Just my $0.02