Really it's down to what you are used to, PIC's are far more popular, and easier to learn (as they have less instructions). PIC's have been about a LOT longer, which is why there is a great deal more information about them - the AVR was an obvious attempt by Atmel to try and capture some of the PIC market, even down to copying a three letter name!.
Performance wise a top end PIC and a top end AVR have nothing between them - and maximum performance is very rarely a concern anyway.
If you are already familiar with AVR's, then I see no reason to change to the PIC, likewise if you are already familiar with PIC's there's no reason to change to the AVR.