Altitude camp in december. Santa Fe?

Santa fe the town is higher than boulder. Both have periods of sunshine in between snow storms, meaning your streets might be clear and you might have 50 degree temps. Both have access to higher elevations. In both cases, those higher elevations tend to hold onto snow a LOT more in those high elevation places, meaning you will be hiking or running with traction devices if you choose to go up.

I tend to run in town in Boulder during the winter, and I very rarely find my road running affected by snow, except right after big events. But to get any higher, you will inevitably be dealing with snow all winter in some form. Whether it's fresh powder, or ice, or sloppy mud. This applies even to tiny local mountains: mt sanitas in boulder, and mt. atalaya in santa fe. Basically, any trail running will be sloppy and inefficient, but any road running is likely to be a-ok and pretty fun.

So is road running in a town at 5400' good enough for your altitude training? If not, santa fe's elevation is 7200'. For regular training in winter, I wouldn't bank on getting much higher than those elevations in each town.

Is this going to be your first time at higher elevation? If so, maybe think about ABQ, NM instead of Sante Fe, NM. The roughly 2000 ft. elevation difference between ABQ, NM & Santa Fe, NM may be the difference between altitude sickness or not. Altitude sickness for sea level athletes does not simply mean being light-headed and queasiness. A sea level athlete is more apt to suffer over-use injuries at higher elevation. Higher elevation inhibits recovery. The benefits of higher elevation for a sea level athlete are simultaneously increasing a sea level athlete's risk of injury. The true benefit of higher elevation are for endurance athletes who were procreated, carried in womb and grew up at higher elevation.

Santa Fe is pretty cold in December and does get some snow. But the really big snow usually up in the mountains. Santa Fe is in a valley even though it is at 7200'. Santa Fe is a funny climate that varies a lot depending on how the flow of moisture sets up from the Pacific and which direction cold fronts come in. December can be very dry and sunny. But I would actually expect Boulder to be a bit warmer on average.

Santa Fe will be cheap the first two weeks of December and then very expensive the second two weeks when all the tourists pile in. That being said, it is way cheaper than many of the Colorado ski resort towns. There are tons of VRBO rental houses in Santa Fe that can be a good deal if split between a few people.

Santa Fe has some nice places to run, but the trails in the mountains can be a bit more technical. The town has lots of great restaurants, art galleries and cultural events. You may also want to look at Taos (bit snowier) and Albuquerque (lower but warmer).

I would do ABQ instead - much less chance of inclement weather. While the trail along the river is only 5000ft, running in the foothills or along Tramway (the road on the east side of town) is ~6000ft. Plenty high enough for good altitude training.

What kind of running do you want to do in altitude?

From what I remember of 9 years of altitude running in Colorado that it is cold and you can only run on cleared roads or groomed ski runs if they still let you.

You can't go up any trails because they are full of snow.

Colorado has a lot of blue sky days in winter. No idea what the weather is doing in Santa Fe.

I would recommend La Paz, Bolivia. It's summer there on the southern hemisphere.

Flagstaff has a lot of pros for a reason: if it is snowy, you can drive an hour or less and get to lower and drier climates. Not just sedona or Camp Verde (which is at like 3000 feet and typically 20+ degrees warmer, so good for live high, train low training in the winter) but you can go east on I-40 and run on some dirt roads below 6000 feet that rarely get snow. Housing costs can be high though.