A fixed scope *can* be made to be more rugged than a variable (that doesn't necessarily mean that a given fixed scope IS more rugged, though), because of the fewer moving parts. Fixed scopes need to be either low magnification (4X or less), or for a higher-powered fixed, you need to be using it from a decent distance, so you have plenty of time to get it on target.
The advantage of a variable is that you can leave it on a lower power for snap-shots or for moving targets, so that you have a nice, big field of view and so the gun is usable at close ranges. If you come across an oppertunity for a long shot from a fixed position, you can crank up the magnification. The downside is that it is more expensive to make a rugged, reliable, repeatable variable scope, so buying a decent one is going to cost more.