B looks like it might be a little better made. It looks like it has two rivets holding the top half of the strap to the flap.
B looks like it'll shed water better also. It doesn't have the exposed edge on the band around the bottom edge of the flap.
The eagle on A is a little sportier
The sissy bar and pad that was mentioned above, is a good idea. You can set another bag on the seat right behind you and bungie it in place.
Don't forget some tip money for the cabana boy