One of the most common places to find MR (miniature reflector) halogen light bulbs in use would be in smaller track lighting. They are excellent and efficient light sources for retail environments. It is well known that halogen light is one of the (if not THE) highest quality man-made lighting sources, and using MR light bulbs will allow you to harness that quality and focus the light in very specific places.
In what way are you using your MR bulbs? Depending on your answer, choose a model that accomplishes your goal the best. Lighting up an entire room, you should consider a flood or even a narrow flood light bulb. If you are trying to highlight certain things in a room, you would want to use a spot or narrow spot. That way you can light the item you want, and not the wall space or other objects around it that you don't want to focus on.
Note: The number after "MR" indicates the diameter of the bulb in eighths-of-an-inch. If you are using an MR16 light bulb, your bulb is 2" in diameter. It is not recommended to switch back and forth, although technically it is possible to do as long as you match the base type and voltage exactly. Just keep the size of the bulb in mind if you want to experiment.