I love writing Supernatural, although like you, I find it hard sometimes. I try as hard as I can to keep it light and humourous as possible because of how dark the most recent season can be. Considering the one I'm working on at the moment is Dean/Jamie, it means that I can focus on the lighter side of the fandom as it is before things start getting really serious. I've never tried writing Sam before, but I can see why he'd be such a great character to use. Generally, I end up defaulting to Dean, simply because I think there's more depth to the way that he reacts to different situations etc. Supernatural is also one of my favourite fandom to read fiction for. *giggles*