I don't understand the cult following for either place. Both are fine. Red Robin has far superior burgers to both places, but it isn't fast food, I suppose, so maybe people don't want to compare.
Most cities have local restaurants that make better burgers than either of those chains.
So what specifically do you like about In-n-Out? They just taste like normal cheeseburgers to me. Yes, better than McDonald's or the other really big chains, but still just an average hamburger. I just have never understood the number of people that really love it.
Some people mentioned 5 Guys earlier and I think 5 Guys is superior to In-n-Out, although I do not like their fries.