Also remember that a moving mirror will move in a straight line to where it needs to get where as a moving yoke will have to twist and may not go in a straight line but will get to where it was suppposed to. Also a moving yoke can typically illuminate alot more due to being able to rotate 360°.
The moving head has most of the body of the fixture mounted on a moving yoke. The moving mirror swivels and twists the mirror to that position given the light fixture itself is housed within a non-moving body. This is much easier to do.
This allows for more gadgets in a body that does not require engineering to allow for this inside of the moving body. On the other hand it results in a little less ability to move in a half circle plus. On the other hand, since it's only a mirror moving, it can often move it faster at a cheaper cost.
For the momey, most of the moving mirror fixtures will be good to start with. As long as you can heft the fixture into place - in the case of some older styles, you get just about what you do with a moving head fixture. They can also be useful for low ceiling height areas.
Is it the best solution, probably not but given a limited playing area such as a stage, such fixtures can often provide sufficient lighting.