Six inch cans are more common, for whatever reason. The difference, for your purposes, is basically cosmetic; 5" cans would be less conspicuous than 6", and that matters to some people. Also, there seem to be fewer trims available for 5" cans.

We are using Juno 5" recessed with the Wheat Haze. They seem to be a good product and have nearly every trim you would want available. If you click on the link and download the spec sheet for the housing you want, it will list all the trims available. I think the 5" are just a bit less obtrusive than the 6". You will need more 4" cans (more holes in the cieling) to get the same amount of light.

I can't imagine that it costs anymore to install a 5" can versus a 6" one, but the housing might be more costly. I bought ours online at Laner Electric: (http://www.laner.com/ ) for less than $14.00 each. Trims vary in price between $6.00 and $50.00

Here is a link that might be useful: Juno 5

From experience, the 6" cans are much less costly, probably because they are more common. We went with 6" Juno's from Lowe's with the standard 24W-WH trim, and the Juno spec'ed 75W halogen bulbs for that combo and are very pleased with the lighting, minimal glare, etc. Did the sloped 6" in the great room. Also used some 4", and one special application for a 5". Got better pricing through Lowe's than a builder's discount got us through a lighting store.

As stated this is cosmetic in looks. If you are over budget, you may want to look and see who in your area stocks Nora recessed. They are more reasonable and have the selection of trims that you might want. You can check out their website to see if they have what you want. Although their 5" trims do not have the selection as the 6" ones, they do have trims in black, white, nickel, copper and some others. Check out NoraLighting.com

Thank you for the responses. Very helpful to know that if I decide to make this spendy decision it will be solely out of vanity, as you have confirmed that cosmetic is the only difference between the two.