Take your dog's distance and range into consideration.
The range and distance of your dog's hunting should definetly be taken into consideration when choosing a dog training e-collar. The range you need may vary depending on the application. For example, a hound hunter may need the range up a couple of miles, while most bird hunters are fine within a mile range. Waterfowl dogs are seldom far from their trainers and may be just fine with a range of less than a half mile, and yard and field trainers can get by with even less than that.
However, keep in mind that usually a manufacture's specified range is based on level ground with no obstructions. Hills, canyons, timber, or dense cover can significantly reduce the effective range of any device. Therefore, always consider a little extra when making your E-Collar selection.