In summer, Amboise is absolutely inundated with tourists. There are only two restaurants in town worth bothering with: l´Epicerie or le Parvis. The rest are only there for the tourists. Frankly I find Amboise over hyped and much ado about nothing - or very little.
Tours is the transportation hub of the Loire Valley and a lively college town. If you do not have a car, it is the only place I would consider staying but only then in the area broadly defined as between Place Plumereau (in Old Tours) and Tours Centre - the train station. Other areas of Tours are not particularly interesting for tourists.
The Office de Tourisme, from where the minibus tours depart is just across the street from Tours Centre. Place Plumereau, in the heart of Vieux Tours, is a relatively short walk away and has numerous cafés and restaurants.
I have heard negative comments about Tours from people who have never really been there and know nothing about Place Plumereau. It´s a wonderful location with a lot of charm. I would ignore comments from the uninformed. You´ll have a wonderful time there.