I assume you have not been to Paris or to Rome.
Both cities will be very nicely decorated and jazzed up for the holidays so both are great choices.
You can spend easily a week in each city and not get bored. Both cities has some excellent one day trips if you feel you want to get out for a day.
I would NOT do both cities unless you just want to tell everybody you saw both cities and not enjoy either. During winter you can assume flight delays so take that into consideration.
My choice is always Paris. It is awesome. Tons of museums, parks, cafes, restaurants and just an overall fun city to walk and enjoy. Hotels may be pricey in both places during that period and probably overall Paris is a little more expensive than Rome but not by much. London, even with a better pound/dollar exchange rate, is still much more expensive.
I would read through the France forum and do a search. There are many threads about what to do in Paris.