As far as technique is concerned, the pocket trumpet is identical to the standard Bb trumpet, with the exception of weight distribution and posture implications.
An extremely small child might be started on pocket trumpet for this reason, since the center of mass of the instrument is held closer to the body. But once the child grows up, or when starting an adult, it would be recommended to use a standard trumpet so the student can build good posture habits to support the instrument properly.
Another concern when buying a trumpet as an adult is that there is a novelty factor to the pocket trumpet. This means that you will be paying more for a pocket trumpet of equivalent build quality than you would for a standard trumpet, but also that you will find a large variety of cheap and cheaply-made pocket trumpets on the market (read: Ebay) that should be avoided.
So, if you don't care about building perfect posture habits, you like the idea and aesthetics of the pocket trumpet, and you're prepared to pay the novelty premium (or suffer a bad-quality instrument), then you go right ahead and buy a pocket trumpet. Otherwise, buy a used student-grade standard Bb trumpet made by a reputable manufacturer from your local music store.