Everything I have ever read or seen first hand, indicates ABS increases stopping distance. In fact, stopping distance should be longer in an ABS car because ABS prevents locking up the tires.
The main point of ABS is to prevent you from losing control of the car, not to improve stoppind distance. In a non-ABS car, if you, for example, slam the brakes hard while in a turn, you lock up the front tires. Well the front tires dictate direction of the car, so you would lose directional control of the car. ABS prevents lock up, thus alowing you to steer while braking hard.
However is just a straight stop, ABS prevents locking up the tires, which increases stoppind distance. Non-abs can lock the tires which will stop you far sooner.
Stoppind distance is more dictated by the weight of the car, the weight balance of the car (front to back), the size of the tires and size of the brakes. A more balanced car, like a rear wheel drive, or mid-engine car, while stop quicker because more weight is distributed on all four wheels. A car with a lower center of gravity will stop quicker too. But ABS does not help here.
I'm not going to debate it cause it really is a minor thing. I still suggest ABS should be least of your worries when buying a car. If anything, it's one more thing that can screw up on a used car, that wouldn't be worth fixing. Having worked at a dealership, I routinely saw ABS cars with little bits of tape over the ABS light cause it cost $1000 to fix the ABS and the car wasn't even worth that much.