I will be traveling with my 2 daughters (ages 21 and 24) from Nice to Venice in late July. We change trains in MIlano. One option gives us 15 minutes between trains and the other gives us 1 hour and 15 minutes. Having never traveled by train in Europe before, which is more realistic?
Also, do you recommend making reservations or will you be able to get seats on the trains without them? (We will be continuing on to Florence and then Rome also by train after a few days in each location.) I have to buy our rail passes and want to make all arrangements at once!!!
I recommend allowing 1 hr 15 min. A couple of years ago I missed my connection in Milan because of the delayed arrival of my train from France and arrived in Venice two hours later than expected. The Milan train station has many tracks and it can take a while to get off the first train and get to the second train (after figuring out where it is). I usually make reservations, but they are not always necessary.
I agree with Swan, allow the 1 hour 15 between trains. Milan is a really busy station and not that easy to find your connection at times. I arrived from Genoa year before last and missed the connection as the train stopped for some reason 10 minutes out of Milan for about 20 minutes. When we did arrive we checked which platform we needed to get to and headed that way. As we were the only people there waiting for a train I went back and checked again only to find the platform number had been changed. I also make a reservation but often you really don't need to.
I will gently disagree. There is frequent train service between Milano and Venezia Santa Lucia. Book the 15-minute connection. If you miss it, take the next train.
The trains you will take between Nice and Venice require reservations. If you have a railpass, you'll need to buy reservations. The same is likely to be true for Venice-Florence and Florence-Rome. The reservation fee for EuroCity trains is €3-€4, but it is €15-€20 for EuroStar Italia trains. If you buy point-to-point tickets, the reservation is included in the price of the ticket.
Will you be traveling by train anywhere else in France besides the short ride from Nice to the Italian border? If not, don't buy a railpass. Point-to-point tickets in Italy are cheaper than a pass. If you buy your tickets a day or two ahead of time, ask for an Amica fare (20% discount). Those fares are sold, subject to seat availability, up to midnight of the day before departure.