What are the differences between the two? which do you prefer? As I imagine these mods would not work together???

No they will not work together in any way, shape, or form. I've tried both, and while both are very impressive, I find I like Realistic Lighting best. Also you may want to look at another mod calledhttp://skyrim.nexusmods.com/mods/1875 (aka RCRN). It also has some amazing features, but as I said RL is my favorite.

All 3 tweak the lighting, shadows, HDR, and Bloom along with colors and other personal things within each mod in their own different way. The best thing to do would be to try all 3 out and see what you like best for yourself. Realistic Lights tends to be the most compatible with other things though.