Either. I use foil because I always forget to buy butcher paper. People will
argue that they can taste a difference, but they're just trying to sound like experts on the subject. You'll get the same results either way.
Here are two pieces of advice that many people neglect to mention.
1. Temperature is important, but it isn't everything. Look up both the jiggle test and the fork test to check for doneness.
2. Don't forget to rest the meat (don't cut it) for at least 30 minutes to an hour after taking it off your pit. It makes a huge difference.
Don't get discouraged if it doesn't turn out the first time. You'll get there with practice.