Portland in 7. Spurs showed their vulnerability in that Mavericks series. The Blazers are hungry and hot.
Only reason I say 7 games is cause of the Spurs experience.
Very simple factor here. Whichever team wins the PnR matchups wins this series. I feel like Tony Parker and Manu Ginobli are in for a huge series, maybe their presence won't be felt on the box score but those two will break down Portlands defense and either score themselves, draw a defender and kick it out, or their penetration will lead to a hockey assist.
I know people love them some Damian Lillard, but watch him defensively in this series. Especially when he's defending Tony Parker, though if Terry Stotts was smart that won't be very often. I wouldn't be surprised if Batum were to exclusively defend Parker. You'll probably see him on Danny Green or Marco Belinelli. Specifically Green as he's not a threat off the bounce (neither really are, both sub 40%) nor is he a threat using ball screens, where as Marco is actually a solid PnR ball handler, though Popp doesn't elect to use him in that role very often. But if Stotts does go Lillard on Parker, you should quickly see why Lillard isn't a top PG in this league.
Same thing goes for the other side of the ball too, as I could see San Antonio electing to put Leonard or Green on Lillard if Parker isn't healthy. LaMarcus Aldridge will also pose a problem if he can hit his shots. San Antonio hedges high on ball screens and that leaves you susceptible to an open mid range jumper, though not as much as hanging back in the paint. I personally think they'll let Aldridge take those long low efficiency jumpers rather than leaving corner shooters open.
I originally picked San Antonio in 5, but after taking a deeper look at the two teams I think the series will extend a little longer than that. I'll go San Antonio in 6.