I own both, and they are both fantastic pins.
Police Force is one of the best "family" games around, IMO. Fun, easy for kids and noobs to pick up on the ruleset, and it plays great. The kids LOVE the flashing police lights, moving car, and sirens.
Taxi is just simply - one of the best System11 pins ever made. I play mine every day. It never gets old. The art, callouts, and classic flow gameplay make it a lifetime keeper for me!
I love them both - but I'd choose a Taxi for my own personal use, and seriously consider PF if this is going to a house with young kids.