Since you asked for opinions, my $0.02 follows.
If I only had one shotgun, an MKA 1919 would not be it. As a novelty or whatever, maybe so. I would choose a more utilitarian style for a first and only. As it will be your only shotgun until you add a second.
Tell pops in law you only have $400 and let him respond.
But we still have a few freedoms of choice left. So you make the call.
Of course, you could get both. By the Mossy and say hey pops, can I buy your shotty on an installment plan?