I had this tyre choice dilema some months ago, and my '28SP was fitted with GPR70's when I bought it last year. After some deliberation I chose to stick with the GPR's as they felt very good in the weather the bike sees - ie. cool - hot temeratures, never in the wet. The well worn GPR70's fitted when I bought the bike gave excellent traction/side grip, warm up OK and have never given me any big scares. I got the GPR70SP tyres which I think are a newer compound or something. They are quite expensive though, cost me £160 plus fitting.