The number you're referring to is the shore or durometer rating. The higher the number, the harder the tire, which normally equates to less grip, but that's not the end of, or the entire, story when it comes to foam tires.
The tire that's going to work best depends on the surface you're running on, your setup and speeds. Run too soft of a tire, and they won't last, run too hard, and it'll never hook up.
From a money standpoint, running on a parking lot track, start somewhere in the middle-high shore rating, and go up or down from there depending on tire wear.
You'll also find harder front tires compared to the rear will balance out tire wear. Ideally you want all 4 tires to wear at about the same rate.
Privateer racing lots of stuff.
Remember kids, racing isn't fair!