mcassel, I think part of the info you're missing is this: The MR in MR16 stands for Mirror Reflector. The AR in PAR stands for Aluminized Reflector. Highly polished mirror reflectors are able to direct light in a very focussed, controlled way, so the light the bulb produces can be directed to in a very specific direction (yielding a high candlepower in that direction but throwing little light off to the sides. This effect is increased with low-voltage bulbs because the filaments are smaller. However, the tightly controlled beams that are native to these bulbs can then be softened and adjusted with optional diffusing lenses that dim4fun mentioned. Aluminized reflectors are much less polished and tend to scatter the light, so less of the light is concentrated in the middle of the beamspread, and more is sent off to the sides. If you want dramatic, spotlight-like effects then the MR bulbs are the way to go. If you want softer-edged light, or if you're trying for even lighting, where the pools of light from adjacent fixtures can blend together somewhat seamlessly, then PAR lamps are probably the better choice. If you want soft light but need especially small or versatile fixtures then MR-types with diffusing lenses might be the solution.

I'm only chiming in here to talk about aesthetics. I have a couple of 4" halogens that provide spot (task) lighting for my prep area - they are fabulous, but due to being installed in a "light bridge", they're only 4' off the counter. I also have 5" compact fluorescent cans in the kitchen and 6" incandescent cans (old) everywhere else. Aesthetically, i like the 5" best -- the 6" cans look enormous by comparison, and you need too many of the 4" cans to do the trick.

You don't. You remove the mini can. Since the junction point to the 120V connection needs to be accessible, there has to be a way to get to it without removing the finished surface. This is usually accomplished by removing the trim piece and pulling the fixture down through the exiting hole allowing you to reach up and get to the junction box. Depending on the design of the fixture the thing may just be friction fit up in there, or there may be clips or screws that you have to undo to get it to release. Any electrician worth a plug nickel will be able to do what I suggested.

Thanks everyone. I should have explained the reason. I needed to install 6 of them in a row with about 2 feet between them. I had already marked out the locations. I bought these recessed 4" kits from Home Depot and bought some LED GU10 lamps for it. http://www.homedepot.com/p/Globe-Electric-4-in-Brushed-Steel-Recessed-Lighting-Kit-Combo-10-Pack-90542/203976636 Installed 5 of them without any issues. However on the 6th one, I found that there was a pipe in the ceiling that reduced the height and it's right above the hole I cut. The distance from the bottom of the ceiling to the pipe is around 4". So I need a can that is less than 4" to fit. I am hoping I can use the same trim from the kit I bought will fit other housing too. Since the rest of the lamps are all GU10, I wanted to stick to a uniform look. Vith - I thought of using the cans you have listed by getting rid of the transformer and then using GU10 bulbs. But I wasn't sure if I am in any way changing what it is approved for and thereby violating any code.