For X-mas lunch it should be like a fairly normal holiday, but Christmas Eve is actually Argentina´s most important holiday, similar to Thanksgiving in the US where virtually EVERYBODY goes home for dinner with the family. ALL malls, and virtually all restaurants close down for the evening.
Another characteristic that reinforces the shutdown is the widespread MIS-use of fireworks by unsupervised kids and foolish adults. You wouldn´t want to be walking in the line-of-fire from 10pm to 1 or 2 am. It becomes a real "ghost-town", the air even gets full of gunpowder stench.
By comparison, New Year´s Eve repeats the taxi, and fireworks situations, but people tend to go to visit friends or out to restaurant parties, instead of family. The city is only half full by then; after X-mas the vacation exodus begins.
Remises and taxis will be hard to find, but you can call in a reservation a few days in advance with a nearby remise or cab agency for pre-arranging drop-off and pick-up times-places. Usually they will ALL be away from 8-9 pm till 1 or 2am, but you would also be having your dinner so it kind of works out well.
I would imagine in Puerto Madero, or in the Palermo area, you would find a Christmas party for tourists and local singles. In the suburbs you have the restaurant strip at Acassuso, by the "hipodromo" racetrack that always has a few open places. But you better look into all this yourself, check and ask if the ambience is of your particular liking, and make your own reservations in person.