Is it a choice between only those two devices? Tell us what is most important to you in a phone and we'll try to advise.
With regard to hands, when you are 13, they usually increase in size with the passing of time until a maximum is reached, more or less. But even for adult hands, some of the large devices now available are too big for comfortable use.
I like the size of both phones, but it's the software and hardware that I'm concerned about. The battery life on the moto is concerning and the UI on the g3 is concerning
Three months is a long way away and by that time new devices will have made their appearance. The Nexus X for example is a much-anticipated device....as is the Nexus 6 Shamu, a monster of a device by almost anybody's standards.