what is the performance difference between brushed motors and brushless motors? why would one choose one over the other?

Brushless is less maintenance, more efficient. Why would you choose brushed over brushless? Only reason would be cost, or your local track has a brushed class.

Brushless motors should be more efficient giving slightly longer run times or slightly more power from a fixed battery size.

Suspect you can get close to the same perfromance from either, but brushed will take more work to get there and stay there with comm maintenance, brush/hood alignment/maintenance and more. Brushless otoh, is basically plug and play to get its best performance.

The reason I run brushed is that they are waterproof. I dont know of any good waterproof brushless systems.

Theres no comparison. Brushless is faster, has more torque, almost zero maintnance, longer runtime, etc, etc, etc
The only thing that brushed is good for is crawling. Thats about it.
You can even get budget brushless set up for the cost of brushed now a days. Hobbypartz and Hobbyking