I, too, have done Atkins. Frankly, I love it. I found it difficult to give up certain foods (ie-junk food!) though, so I am enjoying IP right now because all the choices are made for you. I like the KISS method, don't you?
Long term I know that my diet is going to be mainly low-carb, the same way I have a friend who is vegetarian because she can't process meat. My body does NOT handle carbs (ie-rice, bread, starchy foods mainly) very well at all.
My goal with IP is to lose my weight quickly and go from there. I don't know anything about maintenance, I don't want to know about maintenance until I get there! If I like it, I'll use it. If I don't, I'll try Atkins as a long-term lifestyle. I believe I'll have greater success at it as I'm determined more than I have been before. I know it works, I know IP works, and I don't think one is better than the other. They are similar with different pros & cons, just like any diet program.
Go Atkins! Go IP!