I think they both have their warts and flaws, but both have reasons for praise. Both films move very slowly and focus on building tension. If you're rewatching, you can safely skip the first 30-60 minutes of both to get to the more compelling material. Long dry scenes of a kid on a big wheel? Fast forward. Running through a snowy hedge maze for what seems like an eternity? Fast forward.
I think Linda Blair is a better and more believable child actor. Sure The Exorcist doesn't have Jack Nicholson, but the cast as a whole is more compelling. And ultimately, I think it's the cast that will help the film age better than the effects.