It's a question often asked. Do you provide handouts BEFORE or AFTER the presentation? Someone finally did a study and you can read the full 10-page study by Jennjou Chen and Tsui-Fang Lin. If you want the quick answer, before is better. The study concludes that you will have better learning by providing handout before the learning presentation.
Here's the full conclusion of the study's authors.
"This study contributes to the literature by exploring the link between downloading PowerPoint slides before classes and students’ learning outcomes. The effects of using downloaded lecture slides on students’ academic performance are statistically significant across models. Our estimation results have shown that downloading lecture PowerPoint slides before classes improves students’ examination performance by 4.62 percent in the least squares model. In addition, after controlling for individual heterogeneity, downloading lecture PowerPoint slides before classes still enhances students’ examination performance by 3.48 percent. Also, for this group of students, the lecture slides effect has been found to be greater than the average attendance effect. In light of the statistically significant and non-trivial effect of downloading lecture PowerPoint slides before classes on students’ examination performance, we suggest that instructors could help students improve their learning outcomes by supplying lecture PowerPoint slides before classes."