Paris to nice

Dave anyway you slice it, its a lot of travel for only two days,, I would try and make it at least 3 days since the day you arrive and leave don't count for much. Train is 5hrs 40 minutes.. took the TGV last summer. City center to city center though. I booked through idTGV which is still TGV but since you must print tickets at home its usually a cheaper option then booking through TGV, it is all the same train though( I know seems weird but some cars are idTGV and some are TGV but they are all the same within class) . In the idTGV cars you do have a choice between IZap and IZen. IZap is for families , people who want to chat with friends , those who wish to talk on their phones etc. IZen is the "quiet " cars, families can travel in there but usually opt not to since kids are kids, we took IZen, and found it nice, we still could chat etc, but it was over all a peaceful trip. The flight is about 1.5 hours, but you must get to airport outside Paris, that's 45 minutes, and you must be there 1.5 hours ahead of time. In Nice the airport is about 25-30 minutes from city. So ,flying is a bit faster, but not a lot faster. Flying you must be aware of luggage restrictions, train, anything you can carry in is fine..
Look at Vueling and Easyjet. I would choose what is cheapest and works with my timing best. Last summer I booked Paris to Nice 1st class on idTGV for 40 euros,, we booked the day the tickets became available, we ended up chatting to couple sitting near us, they had paid 200 euros... but had only booked 3 weeks in advance. It pays to book ahead!