I use Leather for Obedience classes and trials, as they look nice Not that the judges take any notice of our leads LOL.

Nylon for getting to and from the park. None when we get there.

I also do not trust retractable leads at all, Clover had one that broke after a few times using it and a friend of mine got one for one of their Boxers and it broke the first time they used it (It was supposed to be good quality) .

My Mums Penny uses one and has been using the same retractable lead for a few years now and it is still going strong.

Leather is still my favourite though.