Take one to be the cornerstone of a team? I'm going with Duncan. Model of consistency, showed up in big moments, great teammate (he never had a problem with Parker or Ginobili and eventually Kawhi taking some of the spotlight).
Shaq in his Magic days was a force of nature and a dominant player when he felt like it in his Laker days, but he was kind of a pain in the ass to deal with and he spent too many seasons using the regular season to play his way into shape. The Lakers, Heat, and Suns all eventually just gave up on him and dealt him away.
Higher highs with Shaq and if you're more concerned with having a guy who sells tickets and ensures you'll get all the choice TV spots it's a no brainer, but you take Duncan and your team is a lock to contend for the next decade, year in and year out and without all the extra drama. And if we're talking about them playing in today's game I think Duncan would be better able to adapt.
Bonus: Duncan was one of the few bigs in the league who could go straight up with Shaq in the post and hold his own on defense. Makes life a hell of a lot easier for his four teammates.