Pipes are like shoes in many respects. What is attractive and comfortable to one person might be all wrong for another.
From a technical stand point, here are a few considerations.
Straight pipes tend to burn a little more evenly, cleanly and perhaps even a little cooler. The trade off is that holding it firmly in your teeth requires much more effort than a bent pipe due to basic leverage. If you get tired of holding it in your teeth you'll need a free hand to hold it. Also, those of you who, over the course of time, have traded in your natural teeth for the man made kind might not have an easy time with larger straight pipes. I find straight pipes with saddle stems to be the most comfortable ... but hey, that's me.
Bent pipes are much easier to hold in your mouth and do not exert nearly as much leverage as a straight pipe. They also allow the smoker to catch an occasional whiff of their own tobacco directly from the bowl. Most people find them much easier to light and keep lit because you can see more of what you are doing. The downside? When holding a bent pipe in your mouth over an extended period of time saliva tends to run down the stem causing moisture in the bowl which leads to a hot, gurgling pipe. Some less expensive bent pipes will not pass a pipe cleaner through without taking the stem off. That's OK when the bowl is cool but a major no-no when the bowl is hot.
I hope this helps ...