What's your reason for selling it? The reasoning is hard to decipher from your post.
You mention gas money and MPG, so I am assuming you want a better car for fuel efficiency, yet you mention you might decide to get another M3? That doesn't make sense.
If you can afford it, perhaps get a second car to drive around in? A hoopty? Do you own the M3 outright? Just take into consideration the cost of selling and buying another vehicle so you don't make a mistake.
Good luck with whatever you decide!