The big difference is how long they will last over a set of stock tires.
On hard packed clay, those things would be toast in a weekend...so $37 seems cheap up front but for as much as I drive, I would easily burn through $100 a month in tires... doesn't sound to reasonable to me. Even 2.2 proline mulchers or something similar would be a more economical choice. I am not a pro racer with tires being thrown at me nor do I care to toss money out for a tire that will help me win a club race... just not worth it.
I will stick to the stockers, they may have poor grip but they last forever and keeps things cheap. The Slash class helps me take a break from throwing money at my hobby.