To me this is like comparing a pickup truck to a freight train.
WSM is a wonderful smoker, so wonderful in fact that plenty of teams use these in competitions. They're light weight, easy to maneuver, easy to use. But, they're really a smoker, and just that. They also aren't insulated. Made very well, and well worth the money.
BGE is a fantastic cooker that can be used as a smoker, an outdoor oven, grill, etc. It's not cheap. eggzlot says 2x a WSM. I'm thinking that's an unbelievable understatement. I think it's more like 4x. They're heavy as **** because they ARE insulation (in essence). They cook/smoke fantastic, and many competitors use these as well. However, you'll see many that have fallen or otherwise broken. Not that you cant break a WSM, but it's a lot more costly to replace one of these than a WSM.
Money not withstanding, I'd choose the XL BGE (or the new XXL BGE) for versatility.
Money, and if I didnt need to cook a pizza, etc. outdoors, and only wanted at smoker, WSM.
Hance - Lake Dogs Cooking Team - MiM/MBN/GBA CBJ and comp cook
Lake Sinclair, GA (strategically about an hour from everywhere)