There are a lot of films listed above, let me try and narrow it down some. Most people want to go with the darkest legal film allowed in North Carolina, which is the 35%. For windshields most go with a very light shade, either 80% or 70%.
The main objective for tinting the windshield is to block as much heat as possible and reduce the glare a bit, without impeding you visibility. As far as XR vs XR+ the main difference is that XR+ blocks even more IR heat.