There are so many choices in materials that your reins choice can be really unique to what feels best in your hand. You may want them long enough to knot them which is a help on cross-country in the event you lose your reinds, it is easier to pick them up and manage them with the knot.
Consider the job of your reins. Many dressage riders prefer the reins stop. Rubber lined on the inside only of the rein makes for a nice look. The stops not only provide good grip on the reins but also help be sure you ride with an even contact. It can be easy to slide an outside rein on a circle and not realize until you are on the long side that your reins are no longer even and the horse is crooked. Rein stops can be reminders: a helpful riding aid.
"I like reins with stops. I want everything even steven." -Shelly Francis
Both are easy to hold if your reins are wet either from the water jump or sweat. And your gloves are a factor too.
Eric Smiley preferred Canvas reins on course at Badminton Horse Trials.