IMHO if they are roller coaster lovers, don't try to convince them that Disney is "better" than their local amusement parks.
I AM a roller coaster lover. I also grew up going to
Disneyland and loved that. When I went to WDW for the first (and only, so far) time in high school, to be honest... it was disappointing.
Why? Because I didn't "get" what WDW was all about. It started, finally, to sink in the night we went to Epcot and watched Illuminations over the lagoon, standing near the Mexico pavilion. At that moment, I was completely and utterly at peace and so happy to be there.
It's sad that it took me our entire trip to even begin to understand. If someone had just said, "No it's NOT about the rides, it's about immersion in an experience" I think that would have helped. Telling me it's BETTER than our local theme park did not help--not at all. Because IMO, WDW is NOT better than our local park (Hershey) when it comes to thrill rides. In fact, it's immeasurably worse.
So comparing the two doesn't work. But if someone had said, "Look, it's a completely diferent experience from Hershey (not better, not worse--just different)--it's about immersion in an experience, it's about stories, it's about visual details"---THAT would have helped.
Anyway I still love love love roller coasters and thrill rides. I also, finally, "get" WDW (I think) and am excited beyond measure to be going back now, as an adult, bringing my dd for the first time and watching her live the magic that I remember from being a child at DL.
But they really are very different experiences. If people who are coaster lovers are told WDW is "better" than their Six Flags or whatever, I think they'll be disappointed... because they'll be coming from a completely wrong frame of reference/expectations. Tell them it's DIFFERENT, talk about details, the visuals, all of that. Tell them NOT to expect great thrills, but if they go, to slow down and enjoy the stories and themes.
Going to WDW expecting thrills is IMHO a really bad idea. It does lead to disappointment. If you can get them to think about it in a totally different frame of mind though... to go for the details....then it can be okay.