Ideally, if considering one's entire travelling life, you will make it to both. I'd fit in whatever works best in your itinerary. Chiang Mai is a city. Luang Prabang is an overgrown small town that can get overwhelmed when the crush of tourists is heavy. However, June is low season in LP so if you want to make the trouble to go there, it shouldn't be too bad.

All other issues (price, time, logistics, etc.) being equal or at least manageable, in Asia I'd tend to choose the place that is in most danger of changing for the worse in the future due to overdevelopment. In other words, go sooner rather than later. By this measure, LP might be the priority. CM has already long ago had its huge growth spurt.