I don't know anything about the Merc stuff as I've never actually seen one, but FP I do know a bit about. If you're not going for one of the FP originals like the tweed one they have now or some of the waxed and plaid high collar versions they have done in the past, go for Baracuta no doubt. nowadays there isnt even a price differential so get the original and get the best.
That said, I'd still favor a monkey jacket over any of the harrington options as I've never been a real fan of them.
