Most of the bigger companies offer free trials of up to a month so you can sample them all and decide which one works best for you.
I've been working my way through a list of free trials at
www.uk-dvd-rental-guide.com for the past few months and haven't paid for a single rental yet!
I think it depends on what films you like, whether you also have a games console, and how many films you watch and how frequently.
To get the best value for money, go for a 2 or 3 dvd at-a-time deal, and get them back in the post pretty quickly.
Hope that helps!