I run Avon Storms on Blackbird in the winter and BT021 in the summer months, I get 10-12k out of bothe sets.
The Avon Storms are a soft and cushy ride, stick like glue in the corners, heat up super fast and rear won't slip on you in the rain when you get slightly squared off which is a problem with the BT021's. Avon Storm are the bomb in the rain and wet roads. The soft ride of the tire creates less road feedback so I tend to scrape parts when pushing them hard unlike the BT021's where you get all the feedback you need, thats the other reason for running the them in the summer.
I have never had a problem with the BT021's but I have had to get 3 Avon Storms replaced due to tread seperation in the last 2 years, 2 front and 1 rear all replaced under warranty.
Just my 2cents