There are a great many belay devices on the market and to the uninitiated it may be difficult to decide which one or type to buy.
Try to choose a design relative to the type of climbing you intend to do and then factor in the usual considerations of price and weight Vs function.
As a dedicated indoor or sports climbing belay device a single tube design may be a simple solution and cost effective. When climbing outdoors a design with two channels is much more suitable as it will allow the use of two ropes for belaying and also the ability to abseil on a doubled rope. The more traditional symmetrical models are great for belaying on fat or chalky ropes as they are smooth in general. Highly recommended are the designs which have two modes of friction, either a smoother side or a grooved side which gives more friction and accomodates thinner ropes. These are slightly more expensive and heavier but easily justified by anyone who has belayed a heavier partner or whilst fatigued. Again the Reverso or Guide plate designs add weight and cost but are superb when multipitch climbing. These can be attached directly to anchors when belaying the second with an assisted braking function and to some extent can be used like a progress capture pulley or an emergency ascender.
Finally the mechanical assist belay devices can help with the intensive nature of sport climbing or when toproping with groups. These help to catch falling climbers and help to hold climbers resting on the rope relieving some of the associated stresses. These are generally the heaviest and most expensive belay devices but come into their own in a single pitch environment and are designed to be used with single ropes.