Field's chances will have been reduced due to the way points are awarded as Bevan will have picked up most points on offer in Wigan's recent matches.
IMO for every spectacular try Bevan has scored another one has been made by Field, but when you score 3 in a game that goes less noticed.
I think Field deserves it this year - just look at the interest he's generated across the whole rugby community and from people who usually hate all things Wigan. That tells you a great deal that he's been the stand out performer.
Croft has been very good too and Welsby also, but for me the stand outs are Field then close second Croft