In choosing a pair of English reins, there are many factors beyond personal preference that might affect your decision. Your choice of reins can be helpful to your style of riding, and even improve your effectiveness as a rider. If you ride for pleasure and on the trail, for example, your reins will likely be a different type than the event rider who jumps over varied terrain and gallops through water. But there are some guidelines to help you choose a pair of reins that's right for you and your horse. Let's get into it.
Firstly, reins are referred to as a "pair of reins" or a "set of reins". This is because there are two reins that go from each side of the bit and connect in the center with a buckle. They are not created with just one continuous piece of leather from one side of the bit to the other. That's why riding instructors say, "Shorten your reins!" (plural). The buckle in the middle can be quite helpful when attaching the reins to a running martingale, for example. It allows you to unbuckle them at the center and feed each rein through the martingale rings.
The most popular and well-known reins in the English horse riding world are laced reins. To create great grip anywhere on the reins, a thin piece of leather continuously wraps around the reins resembling a braid. 
They are most often 5/8" in width which fits most rider's fingers and hands very comfortably. It is the choice of riders in all disciplines - trail, pleasure, hunters, jumpers, eventers, and dressage. Laced reins are used in the hunter ring almost exclusively, and also in English flat classes at English/western shows and in the English flat classes at breed shows.
The classic plain reins are just as they sound - smooth plain leather. Plain leather reins are used primarily in the English world when you need two sets of reins. For example, a double bridle has two bits; the small snaffle bit called a bradoon and a curb bit. The two bits each need a pair of reins requiring the rider to have two sets of reins in their hands. This is also the case when using a pelham bit. The pelham bit has two rings to attach the reins as well. The upper ring is larger, imitating the ring of the snaffle bit, and the lower ring is smaller, simulating the curb bit. The plain reins allow you to easily adjust the length of each rein and they can easily slide through your fingers if needed to lengthen one rein or both reins.
The different sport specialties of the English riding world do somewhat dictate what reins you should use. For example, in the hunter ring laced reins rule the day. They are very traditional and have great grip and flexibility. In the jumper ring, you'll see a wider variety of reins including rubber, rubber lined and even cotton web.
For jumpers it's whatever works, and often the extra grip is needed for control at speed. In the dressage arena, black reins are dominant as the dressage world has embraced black saddles and bridles. But the dressage rider's preference in the grip varies from laced reins, to rubber reins, to rubber lined leather reins, and cotton web reins with stops. Grewal Equestrian's Rubber Lined Leather Reins with Stops
are popular with dressage riders to improve the solid connection with your horse while keeping him on the bit. Eventers often use rubber reins or rubber grip reins as they gallop through the water and need the grip even if the reins get wet. Like jockeys at the racetrack who almost exclusively use thick rubber reins, eventers appreciate the firm grip at galloping speed. For any specific sport, always consult your rule book to insure your choice of reins are permitted.
It is most desirable to match your reins with your bridle color and style. If your bridle is made of havana brown leather then your reins should also be havana brown leather for example. 
This is why almost all Grewal Equestrian's bridles include matching reins like the Arnold Square Raised Hunter Bridle comes with the matching havana square raised reins. The Augusta Figure 8 Bridle comes with a matching set of rubber reins - the preferred choice of event riders who often use the Figure 8 Bridle in training and competitions. The Berlin Dressage Bridle includes leather reins with leather stops. These reins are often preferred by dressage riders.
Every rider has a preference for the width of the reins, but the most acceptable rein width is 5/8". Most rider's hands are very comfortable with this width, and it provides a secure grip without being too wide or too narrow. Most reins are sold in horse size, and are 9 feet (54 inches) in overall length. This normal horse length of reins suits most horses and riders. If there are different lengths offered you can choose between horse, cob size, and occasionally oversize. Cob length is 2 inches shorter in overall length, and would be suitable for a horse approximately 14.2 to 15 hands. Some reins also come in oversize,10 ft in overall length, for horses approaching 17 hands or better or for really long necks.
Your choice of a pair of reins is certainly a personal preference for training, trail riding and pleasure riding. You can always have a second pair for competitions when needed. The best choice for you and your horse is a pair of reins that you feel comfortable holding and has the secure grip to safely enjoy the ride!