I spray almost all my bags and use either AppleGuard's (the one in the aerosol can) or Shining Monkey. Knock on wood, I've never gotten a water stain on any bag. A lambskin Kooba is by far my most delicate bag in terms of how the leather needs to be babied, but I've been caught in the rain with it and once the water dries, it's still perfect.
You can also try a forum search as there's lots of info on TPF about leather protection and waterproofing. It can be overwhelming, but I found it really helped to read first-hand experiences rather than relying on manufacturer claims.
Most importantly though, enjoy your new bag!!!