avon is my 1st choice tyre, i just like them... my last set came in about £15 dearer than the pr4 (think it was the 4) but there would have to be a big margin to change my mind. that said, any decent brand tyre should do the job.
as for mixing tyres, due to an unusual series of events i ended up with an avon on the front and 2nd hand pirelli on the rear for a while, didn't notice any problems but at new tyre time it was a new pair of avon's again ... just more "professional" looking i think.
tyre prices are all over the place at the moment.