As an owner of both the #1 difference for me is portability. I don't have the option of taking my XL with me to comp's so I bought a WSM and OTS so I can accomplish smoking or grilling via portable units. #2 is cost- XL with accessories will run somewhere around $1500 after taxes. WSM+OTS is ~$500 so you can also decide if that extra grand is worth it to you. #3 is maintenance- The egg has moving parts that can wear out/break and a seal that is a pain to replace and keep from burning out. The WSM doesn't have any hinges or seals to replace so it is cheaper to maintain. #4 is fuel consumption- The BGE is the most efficient cooker in my opinion it uses VERY little fuel so you will save some money on lump charcoal costs in the long run. #5 is quality of cook results- this should probably be number one but honestly they are so close it won't make a huge difference which unit you pick as they both turn out excellent food. If you love a deep smoke ring the WSM will give you a better result as the BGE has a lower oxygen environment and less conducive to the reaction needed to make a ring deep in color. You will get one in a BGE but it won't be as pretty, at least that's been my experience. Just my 2 cents... Good luck!
FEC120, XL BGE, DigiQ DX2, UDS, Cimarron, Weber Jumbo Joe, Grill Grates, ThermoWorks RT600C