The question is: When should you hand out your presentation to the audience? After or before the actual presentation?
The short answer is: Never.
The longer answer: You should give a handout written in normal readable text instead of handouts of the presentation slides. I’ve often made the case that you should write one-page handouts instead of printing your slides (I’ve even written several posts on how to create handouts).
If the slides are self-explanatory and the audience can understand the whole content just by looking at the slides, why do you need to see the whole presentation? On the other hand, if you use slides effectively and only use them as a visual aid instead of the “base” of your presentation, then people shouldn’t understand the presentation just by looking at the slides. Handouts and slides server 2 very different purposes. Mixing them will create an ineffective slideument.
So, I strongly recommend that you write a one-page handout which summarize the presentation. And even better, you can add further/reading, some references and even your contact details on there, in case the audience wants to find out more. The information can fit on one page, reducing clutter and even more paper stuff for the audience. A great combo-package. When creating a presentation, I actually even first start with a handout draft, writing down the storyline of my presentation, before I start with the slides.
To the question: WHEN? I answer: After, always after. If they already have the handout beforehand (main points, summary, conclusion), then the audience’s interest for your talk decreases significantly.
And if you still insist on giving your slides instead of a summarized handout, then also: Always After.