Buying the reservations is mostly possible only 3 months in advance, in some countries even only 30 days before..
In December is the annual timetable change, so it might be less than 3 months that you can make reservations.
But don’t worry, if you book them one month ahead it will be no problem.
I’d definitely recommend the train, it will be more relaxed and you can enjoy the scenery without stress.

Reservations for the new timetable year (from 11 December) will probably become available in the course of September and October, depending on the railway company. Italian railways will probably again be one of the last ones to publish their new timetables.

Not only Italian but also Spanish… 

Renting a car for INTernational has astronomically high surcharges here in EUR. Plus you are not used at all to how traffic-and drivers- behave. And DEC means its winter-can be freezing cold here and there.

What is often even cheaper as just the various surcharges for taking some superfast long-dist. trains is flying on low-cost budget airlines-except if you also schlepp around a few tons of luggage. But that is very much against the current green spirit. Other as in US or on Aloha this also means booking long time advance and precisely knowing what you do-it is about impossible to even buy tickets on the spot on airports.

Flying through Europe is the most boring thing that you could do… beside the environmental aspect, the quality of service is terrible, you will miss out the scenery along the way and have just some sterile airports on your way…