It's Inferno for me, it's an alround great ride, nice and smooth and has some great elements to it. Colossus is riduculous, mainly because of the cars, in my eyes if you have to crawl to get into the seats, there's something wrong, they're far too cramped it's worse than sitting on a plane. I don't really care about the roughness of it, I'll ride anything but the inlines are just not fun, yes on your first ride theyu were quite amusing and unique but once you've been on a few times it just gets far too b oring and repetitive.
When I visit Thorpe I always make sure I ride Inferno, but it doesn't bother me if I miss out on Colossus, especially with all the queues and breakdowns it has.