I've seen every Hobbit movie. I've seen every LoTR movie. I adore them. They capture the world of Middle Earth almost perfectly, and the soundtrack is to die for. I also own at least one copy of all the books, first read the Trilogy over the span of a rainy weekend. However, Lord of the Rings and the Hobbit are nothing in the story that is Middle Earth. Read the Silmarillion folks. That is the story Tolkien was telling. The War of the Ring is a tiny segment of a much, much larger history.
I grew up with Potter. At eleven, I was heartbroken when I didn't receive my Hogwarts acceptance letter. I still read far more Harry Potter fanfiction (and no, not that kind) than is healthy. The world of Potter captured my imagination like none have before, or since. Yes, it's not the only world to have drawn me in to a probably absurd degree. But it's the only one that I've managed to stick with for my entire life.
The only ones that come close are Star Wars, Bionicle and Doctor Who. I've fallen out of Star Wars in recent years, due to the ever-increasing politicisation of the franchise, the decrease in stories of actual good content, as well as the de-canonisation of the EU.
With Doctor Who, it's only been in the past two years or so that I've really immersed myself in it, in part due to the "loss" of Star Wars. This was also the same time when I started to get into Warhammer 40,000, Star Trek, among others.
And well... I don't think I have to explain why my interest in Bionicle waned.
Potter... My interest in Potter never really waned. It's only increased over the years, and yes, it's had it's ups and downs, but it's been a constant for pretty much all of my life. Even today, Hogwarts is one of the fictional settings I want to live in most, even though I would in all likelihood have less chance of passing my OWLs than a Squib.
There is just something to Potter that LoTR never had for me. A certain magic, if you will. Middle Earth fascinated me as something to study, a history to unravel, but Potter... Potter fascinated me as something to dream about.