How to choose a pipe: material of the stem
Pipe stems (the thing you put into your mouth) for widely available pipes are made of either vulcanite or acrylic. They look similar (usually black in colour), but acrylic is shinier, colder on touch and feels much harder in your teeth.
The reason for this difference is that acrylic is more of plastic, and vulcanite is a vulcanized rubber.
Vulcanite (although treated to almost a density of plastic) is a little softer and feels much better between teeth. It also makes clutching the pipe in teeth much easier. This in turn will allow you easier handling of heavier and bigger pipes.
Heavy pipes with acrylic stem virtually impossible to clutch between teeth (if you are not a shark, of course).
The only drawback of vulcanite is that it oxidizes over time. This leads to some discolouration on the part of the stem that you hold in your mouth. This discolouration can be fixed by re-polishing the stem, of course if you do care about it. I do not.
To me, everyday benefits and comfort vulcanite provides, overweight this minor imperfection.