Short answer - CREE CR6 and the equivalent EcoSmart branded versions can be used in either retrofit or new construction cans.

You will be using new construction cans.

The setup for CR6 is
CR6 lamp (trim is integrated)
New construction recessed can housing

For CFL
New construction can housing with integrated ballast
CFL bulb
Trim

For traditional recessed cans
New construction can housing
Bulb
Trim

Thank you so much.

Is there a large price difference b/t CR6 and traditional-- and is traditional incandescent or halogen these days?

I don't like the CFLs I've seen for this application-- dim and cold.

Every time you consider cost, the total cost has to be considered.
Cost of the recessed housing. This could range from ~ $10 to > $50. The housing for the CR6 should cost between $10 to Cost of the bulb / lamp. The EcoSmart version could be sold at your local home depot for Cost of the lamp/ bulb replacement, frequency. For CFL cans with the integrated ballast, ballast replacement should also be considered as access to the top part of the can is required.
Cost of the trim piece. Like the recessed housing, this could also range from $10 to $40+.

In some jurisdictions (e.g. California), local energy efficiency laws exist to severely curtail the use of incandescent lighting (including halogens). Direct insulation contact and air tight cans may also be required. Check with your city.

On a related note, the phasing out of incandescents has already begun.

The upfront cost of the complete EcoSmart CR6 unit (can + light) could be >$10+ more than an incandescent bulb, can and trim.

However, even without considering local energy efficiency laws and the mandated phase out of incandescents, the energy savings will make up the difference within a couple of years.