If you do not have a car, I would not even consider using Amboise as a base. It´s a small community with very limited public transportation.
Unquestionable I would stay in Tours. There are a number of hotels between the train station and Place Plumereau in Old Tours. They are all within an easy walk of the station.
Best Western Central
Hotel Oceania l´Universe
Hotel du Manoir Tours
I should also recommend an excellent chambre d´hôte or B&B
La Maison Jules
I would also obtain a copy of the Michelin Green Guide for the Châteaux of the Loire.
You can book a number of minivan tours from the Office de Tourisme located just across the street from Tours Centre, the train station.
There are several châteaux which have relatively easy access via train such as Chenonceau and Château Blois. Tours has a number of restaurants of note, museums, shopping, and cafés. However, you´ll want to stay within the areas of Place Plumereau, Tours Centre, and the Hôtel de Ville. Other areas of Tours will not be as interesting to tourists.