I only have experience with Nutrisystem and Medifast.
Even though the Nutrisystem food itself costs less than Medifast, it cost more in the long run, because you have to add fruits, veggies, and dairy to every meal.
Nutrisystem's dinners are more like real food than Medifast. But after a few months, I was DYING to eat real food that I cooked. With Medifast I get to eat real food once a day, and I have lots of good low carb recipes from my South Beach Diet cookbooks.
I lasted on Nutrisystem a few months, only lost about 15 pounds, and then got completely frustrated with the food and the slow weight loss. I've been on Medifast since April, have lost 70 pounds, and am still motivated to keep going.
Forgot to add - you can order Medifast 2 weeks at a time, but I think you'll have to pay for shipping that way.
Good luck with your choice - I hope other people have more input for you.