Your answer could depend on whether or not you intend to put them on "Top" or "Bottom" of the tower top tube(s). Most have them on the bottom but I'm seeing more and more on the top.
If your situation dictates "Bottom" due to storage or low bridges, then pick the tube that gives you the most head room, i.e. you don't want to be standing up from the drivers seat and bang your head (usually "Rear" tube is better).
If you decide on the "Top" of the tower, then definetely "Rear" tube.
Good luck, show some pics when you're done.