I will second the notion that your robot's center of gravity is a major player in how easily your robot will tip over, and can play a role in how you design your drivetrain.
In 2013 our team opted for a climbing design (
http://i.imgur.com/mUfHf6L.jpg) that ended up having a high center of gravity because of the hooks. Even with a "long" style drivetrain and belts, I would end up using the hooks like skids when I rapidly switched from forwards to backwards. While the hooks prevented us from fully tipping over (I could right it by accelerating forwards again) we would have been in trouble without those hooks. The weight was very high up and wasn't centered in the robot, making it easy to tip that direction even with a long-style drivetrain.
Fast forward to 2014, where one of our design focuses was a low center of gravity after last years robot. Our current robot (
http://www.chiefdelphi.com/media/img...02d42d06_l.jpg) has a very low center of gravity with only the collector extending above the lifting handles that are a part of the side panels. The collector is made of box aluminum tubing, meaning the whole collector arm assembly weighs under 15 pounds. With a total robot weight of 110 pounds, that means only 13% of the robot weight is more than 3" above the bumper zone. This robot, which has a square drivebase and frame, was impossible for me as a driver to tip even with other robots "assisting" me in my acceleration.
You do have to design with turning in mind when you have a long style drivetrain. You need to remember that your center wheel will likely need to be dropped more (assuming you use 6 traction wheels or tank treads like we do) than it would be in a wide style drivetrain just to allow your robot to turn more easily. Otherwise you can actually get into a situation where you cause a Jaguar to draw more than it's rated 60a of current and have it reset on you just by turning the robot.
Just a couple things you might want to keep in mind when designing if you're worried about the tipping aspect. As far as turning goes, a long style drivetrain will generally not turn quite as well depending on a large number of factors (how far is the wheel dropped, how "long" is it, how heavy is it etc.). Our team has found that a dropped center gives us the mobility we need, but different teams have different needs.