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.

Why are you doing this? Do you know that they make Par 30 in different wattages and beam spreads as well as more energy efficient 45 watt that can replace higher wattage lamps?

Go low voltage. Yes, they're more expensive, but you get more. They last longer, put out less heat, are more efficient and give better light than incandescent. Don't forget that a 4" hole in your ceiling is visibly much nicer than a 6". The low voltage halogen bulbs send most of the heat up, into the can. Line voltage bulbs send all the heat down... to the top of your head. That's why, if you look at the cans, the low voltage ones are much bigger- even though the bulb is so much smaller. I have a low voltage halogen bulb which lasts 20,000 hours. That blows away any fluorescent. And very soon there will be LED replacement bulbs/ modules for the low voltage fixtures. I wouldn't combine the halogen with incandescent. The incandescent has a different color temperature and is bigger (and not as bright). And when you dim a halogen, the light becomes a very soft and inviting. Keep in mind as well that an incandescent bulb continually emits less light. As the filament burns off and settles on the inside of the bulb (the black stuff you see), it blocks light. The halogen doesn't do that. The burned off filament keeps recycling back until it finally dies. That's how it lasts so long. So at day one, the halogen is twice as bright. 2 years into it, the halogen can be 3 times as bright- simply because the incandescent is dimmer.

I have installed 13 Juno 4" low voltage Halogen lights in our kitchen remodel. There are two in front of the rangetop, one on each side over the counter, one in front of the frig, one in front of the double ovens, one to the left of the sink, two over the sink, two to the right of the sink over the counter, and another two in front of what will be a simulated hutch. There are also 9 more in the family room and three in the hallway. Ha, I should own stock! The LV Halogens are not cheap, costing around $40 each as I recall, plus the trims at $27 each, but IMO they are well worth it in the grand scheme of things. I have installed 50W, 36 deg Solux bulbs running at 4100K in the kitchen area. They are very white and make everything sparkle. I could never go back to the normal yellow looking Halogens. The family room has 3500K bulbs which are a bit warmer looking. The main advantages to the LV Halogens are much more light per Watt and MUCH less heat. You can put your hand right up under them and barely feel the heat. I have all our lights on magnetic LV dimmers. I'm using Leviton Acenti. When I have them turned down really, really low you can occasionally hear a slight amount of hum from one or two... but the sheetrock isn't even up yet, so once that's in I doubt we will ever hear anything. The light you get from these small Halogens is not very diffuse, which is the way I like it. You have light where you need it but the whole kitchen isn't overly bright. We'll also have under counter lighting and three pendants over the bar stool area.