I would go with the modern one. I like how it's clean and silver and doesn't look so dirty c: Depends what your town looks like though and where you want to place it...
I chose the classic one because Booker was in charge of the lost and found and checking if there were any visiting animals (like Saharah, Wendell etc.) in Wild World and City Folk. It just felt right to have him do that again in my New Leaf town.
I also like the look of the classic police station more than the modern one.
I have two towns, one has the modern station and the other has the classic station.
The one with the modern station is a more serious, down to earth town. A competent policeman would be expected there.
The other town has more of a horror movie gimmick. The design of the building itself fits with the general vibe. I also appreciate that the sole policeman in the town is bumbling and inept at his job.