I think, and I am not 100% sure..but all NFL teams have gone mobile - only entry..no printed PDF tickets. They do this for one reason, and one reason only: to drive secondary through Ticket Exchange
Through the NFL Ticket Exchange tickets can be instantly downloaded when purchased. NFL makes cash on every ticket sold on ticketexchange, as well as set minimum prices. A lot of people only like the idea of instant download tickets, they do not want to wait.
Tickets on StubHub will not be available for instant download. Ticket seller will need to send email with tickets to ticket buyer. Stubhub will guarantee a 24 hour window for that. Lots of folks will not like that idea. Drive more business to ticketexchange..and Stubhub I think has a minimum of $6( at least for Sabres games)..NFL wants no part of seeing their tickets sell fro $6.
Bottom line, either service is fine, just be aware that Stubhub could be a 24 hr window to get tickets.