One thing I will say about The Smiler is that it doesn't leave me wanting to immediately run back around for another ride whereas Nemesis is just short enough that it leaves you wanting to get off and join the queue again.
Before anyone gives me grief, that's not a bad thing, as Smiler does feel like the full package. Because you have the projection mapping room, longer ride with three sections to it, the fun house style exit and the shop, I got off it and didn't feel like I'd need to do it again.
Think of it as if Nemesis was a one hour programme that relies on you being hooked at the end so that you tune-in the following week to carry on and The Smiler is a film that you really enjoyed but wouldn't be bothered about watching it again so soon, or if a sequel took a few years to be made.
Some will prefer a more complete experience and others will prefer the attraction that cuts things off just at the right time when you're at the edge of your seat. Smiler ties everything together at the end, but with Nemesis you have to go back and rejoin the queue!
That's not to say that people won't get off and want to run around for another go on Smiler, just as there will be people who can watch the same film at the cinema with different groups of people!
But like I said, that's not a bad thing, it's just another difference I wanted to point out between The Smiler and Nemesis, and the film/programme comparison seems the best way to get across the point I'm trying to make. Though both attractions are roller coasters, they're both very different in the way they entertain you.