Most online DVD rental services offer a variety of plans at different price points. They may offer a monthly plan that includes three rentals at a time at one price, and a more expensive plan that allows for four rentals at a time. If cost is an issue for you, then the price of the plan that best suits you could be a major factor in determining what service you choose.
There are thousands of movies available on DVD, and countless titles are constantly going in and out of print. It's impossible for a service to have every movie you want, but some will probably have more than others. Browse their selection and see whether they are lacking in a certain genre or type of film. Maybe one service will have more educational titles than another, or perhaps more cult films.
Many online DVD rental services offer additional services. These can include instant streaming through a computer or video game console, or video game rentals. These services may be standard across all service plans, or may cost extra. Determining whether you want these additional features should also be a factor when choosing an online DVD rental service.