Ideally, neither location would be my choice...
The best location for all the fittings is the top of the tank, with a pickup tube inside the tank on the discharge. However, because that requires at least 5" clearance above the tank, that's not always possible. Although the discharge fitting on all "off the shelf" tanks is on a wall AT the bottom, your idea to put the discharge ON the bottom isn't original. Check this out: http://www.sailcopress.com/editorial%20MSD.htmI don't like the idea much, 'cuz although it does allow the last ounce of waste to be pumped or dumped, unless you're scrupulous about flushing out the tank after almost every pumpout, any sludge in the tank will dry out and turn to concrete, or be packed into the discharge hose to clog it.But there's nothing horribly wrong with it. So where you put the discharge fitting is up to you.