Hot Wheels and Matchbox are both diecast car brands owned by Mattel, they are roughly the same size around two and a half to three inches long at a nominative 1:64 scale. Matchbox is older having begun in England in the fifties, Hot Wheels came onto the scene in 1968 from the USA, with their fast wheels on piano wire axles. I prefer Matchbox because of That Nostalgia Buzz they were the toy cars I grew up with. However, I have more Hot Wheels in my collection because they are far more numerous and have a better distribution network. Matchbox tends to be more focused on realism being copies of the cars and lorries (Matchbox has far more lorries than Hot Wheels) that can be seen on the roads. Hot Wheels has more fantasy elements and the cars tend to be slightly modded or hot rodded from the base car.
I think the difference between the two brands is nicely illustrated by these two Minis, the Hot Wheels model (left) has a lot of graphics, a bump on the bonnet and is shorter and wider. The Matchbox model is more like a Mini you would see on the road with simple graphics and more realistic proportions.
I will continue to collect both brands.
Below are two Mustang Coupes, on the left is the Hot Wheels model, which has larger rear wheels and a slightly more custom look than the Matchbox model to the right.