this is my vote as well you have lots of vertical support on the ends but I would definatly go wiht thicker for the top horizontal. either that or add a 2x4 center support and you would be fine.
You could park a mack truck on any of them. Toe nailed 2x4's with a 15" center even "A" is alot more than you'd really need especially if you plan to case it in plywood.