"To prefer" is a way of discussion but ...
Personally, I like both processors. Right now I just learning PICs but not in an exclusive way (I have an ATmega32 which I bricked already - don't laugh - and I want to unbrick it using PIC18F2550 and JAL language).
"Benefit for PICs users"... well, as I said, we have our own Arduino boards and we can use Arduino shields and also our own.
Is a good way to combine both worlds of fans. AFAIK, Massimo Banzi is an "old" PIC and JAL programmer. In those early times, JAL was a good compiler but lacked many features of modern compilers. Not the case anymore.
There, at Jallib project, we have JAT (in develoment right now but already good results), some kind of JAL to C translator (in fact, is more than that) which permit to program in JAL and target many families of processors. AVR, ARM, etc...
As you can see, we (well, some of us) are multiplatform oriented.
And not the last, many beginners find PICs more accessible and start with PICs. Why not have this bridge between the worlds?
Vasi