Cool topic, I'm definitley with those who say that 20mm toy cars are the way to go. I'm really hoping to try my son on car gaming when he is old enough, though I'll probably get back to work on my own very-simple rules that have been shelved for a year or so now.
Did need to address this though...
72nd scale, 20mm, HO, matchbox, hot wheels, all the same scale. You can get slightly beefier cars with Jada or other brands, and the sky is practically the limit.
Not quite the case. The are all fairly compatible, except for HO. You can use HO scale buildings and scenery, but HO scale cars are drastically smaller than Matchbox/HotWheels. HO scale figures are also notably smaller than 1/72nd.
Just didn't want you to order any HO scale vehicles thinking that they would be in scale with Hot/Match cars.