When I was a child I kind of loved Mewtwo and thought it was the strongest Pokemon ever. At that point, I didn't know about Mew's existence. When I heard about Mew I was amazed, it sounded like a very rare Pokemon and hard to get. I think I spent a lot of time trying to figure out how to catch it. Eventually, I gave up and I didn't find out the glitch that allowed you to catch Mew in Red/Blue till a few years later. When I caught Mew I wasn't as excited as I was the first time I caught Mewtwo (maybe because I was a child and I thought a 7-level Mew was weaker than a 70-level Mewtwo? I don't really know).
Though I like Mew more than Mewtwo right now, I'll go with Mewtwo because it meant a lot to me more than Mew.