I know your pain in deciding - at this point I've got a single BR strap, but may go forward with the Spider simply because if I have a back pack on, I don't want to have to take it off to get out of the BR strap (you'd have to wear it under the backpack. The catch is that what ever pack I use, it can't have any type of belt on it. Now if you are in a place that's really warm, the idea of wearing a belt may not mesh with your style or comfort. Either one is going to suck if you're crawling around to get a shot.
Questions I have that would impact this include, what lenses are you taking, what about a second body, are you using a backpack or such, are you going alone or with a partner, where are you going to be - are there local security concerns, weather of where you'll be - rain protection. There is no magic bullet for possible solution, and you may want to just purchase both, so you can decide later.