Venice first week in December

Well it's probably going to be darn nippy in either place. Probably more rain in Venice. Still, both are unique and enjoyable although Rome probably has more to offer in the sheltered out of the elements category (discounting the Forum etc.) If you really want to go to Venice I say do so. Dress in layers and have a good, breathable rainproof jacket and some waterproof pants (the kind with a zipper at the ankle so you can easily slip them on over your boots). Pants weigh next to nothing and if you never use them oh well. I've worn them just for extra warmth and then taken them off once inside the museum etc.

Rome on average is wetter than Venice in late November-early December: over 4 inches per month for Rome versus 2.75 inches per month in Venice.Rome is slightly warmer than Venice that time of the year but both are coolish. Typical December temps in Rome are high 58 low 40 (these are the average high and low temps for December days).The same temps for Venice are high 48 low 35, a little cooler on average.These are long term averages, temps in any particular year may vary, sometimes substantially, of course.

I think if you really want to go to Venice, then do. Maybe try to imagine how you will feel, when you get home after your trip, if you don't go. Will you be disappointed? What if you never get back to Italy again, will you be sorry you missed Venice?
Good luck with your decision, whatever you choose to do, enjoy!

I agree. If you want to go to Venice, go. I would choose it over Rome. Weather varies so much in the Mediterranean area that one can never predict. I spent several months there and can remember many day of bright sunny weather during Nov and Dec.