It's all about efficiency. Basically if you put more winding copper into the motor it will be able to produce a given amount of torque more efficiently than one with a small amount of windings. However more windings will also make the motor much larger physically. Which of these is important to you really depends on the application. For example, an air conditioner motor will have more windings because efficiency is more important than weight, while a model aircraft motor will have less windings and trade efficiency for weight.
In terms of reliability the main issue is heating in the motor. The smaller motor will simply get a lot hotter and also have less mass to absorb the heat. If you don't deal with this heat properly then it could fail.
The other aspect is the motor RPM. If you can run a motor faster it is generally more efficient. However at some point the motor will either fly apart, or core losses will start to dominate, reducing efficiency again. In a well designed motor these points will all converge at the max operating speed (hopefully with a bit of safety margin on the flying apart RPM).
In terms of assessing this information you'll need to look at the rpm/voltage or torque/current constant and the winding resistance. Using these two numbers you can work out the efficiency of your system and decide what sort of efficiency/size tradeoff is appropriate for your application.