The 3 mm thick filament used in the recent years is the standard thickness, as the conventional 3D-printers only processed this size. Even today this strength is widespread although to the new thickness of 1.75 mm can be ascribed more benefits.
Firstly, the slightly thinner material can be processed faster because the 3D-printer requires less time for heating the material. In addition, it has the advantage that less pressure has to be applied to push the material out of the printer, which protects the components of your 3D printer. Also, the stock-feed is precisely adjusted and the smaller diameter of 1.75 mm is more flexible. The 3 mm filament, however, has its advantage as well. It is available in a number of variants, which can be attributed to the higher spread. Thus, you should first make the choice, which filament is best suited for your project before you decide on a 3D printer.