On a Plane:
For plane travel, ideally your piece is small and discrete making it easy to pack in a suitcase or bag. When space and weight is limited, having a compact piece is extremely important. Having a piece with an aspect of durability can also be useful, but even glass can be wrapped properly for airport transport. And yes, you can bring a glass smoking piece through airport security!
On a Train:
With limited luggage space and potential for rough handling of your bags while on a train, having a compact, durable piece will be the easiest to safely transport. A lightweight and easy-to-store piece will make train travel a breeze.
On a Bus or Car:
Depending on the specific situation, bus or car travel may allow you to bring a bigger piece due to having more available space. This opens up a variety of fantastic travel pieces to effectively bring on the road. Smaller pieces work wonderfully as well as they are extremely easy to toss in a bag before you depart.
On a Boat:
On a boat, having an easily holdable, easily packable piece is key. Being able to quickly store or hold onto your piece in the face of big waves is an absolute must. An overboard piece is lost to the water and makes for a bad time. A degree of durability is appreciated too, in case anything falls onto the boat.
When traveling on foot, having a small, pocketable piece goes a long way towards quality of life. Ideally, the piece is lightweight and easy to store to keep your hands free and easy to reach when you need it. Being able to utilize your pants pockets and outside pockets of any bag you may have allows for extremely easy use.