Depends on weather, interest, etc.
Road Greetings "C",
Boston to LA in early December is no problem. We have done similar routes in all months of the year. If you were manage to hit an early (blizzard) snowstorm in Denver it wouldn't matter if you were driving a tank... AWD vehicles are fine in most snow storms but if the road is closed--- Denver rarely gets much snow until the end of January.
Staying in large chain motels will not increase or decrease safety issues -- it can lend an air of normalcy, but the best roadtrips are those where the unexpected happens. Grand Canyon in December could be closed due to snow. Seven days is great -- I have made the same trip in about 60 hours -- but I wouldn't recommend that!
If you want the fastest route -- less affected by weather go I-40 or I-10. Both routes are very likely to have bad roads in Texas due to ice -- but the rest of the trip will be realtively trouble-free.
Have fun, traveling in December is a kick!
RW