I've always thought it'd be interesting to see a cop in a mtb chase a cyclist on a road bike. I'd put my money on the road bike.
I would give much better odds of the mtb cop catching a suspect on a road bike than a cop on a road bike catching a suspect on an mtb.
You only need to be a little stronger on the road on an mtb to catch the road bike. But if you are on a road bike, and the suspect on an mtb, all the suspect needs to do is hop a couple curbs or shoot through some green space to lose you.