I'm the other way around. X is the most terrible Final Fantasy in existence, tied with title of "the worst" in the main series with VI, whereas I could actually be bothered to finish XIII, which I couldn't with X. XIII is okay, not too high on my list (all my favs surpass it, and it's not one of my favs). X is terrible, and is a game I wish I could forget I played. It had some good things (good battle system, some nice music), but most of it sucked (story, characters, sphere grid, character design, VOICE ACTING!!!). *shrugs*
As for similarities, oh they're there all right. Saying they aren't there is simply being in denial. Of course, there's some big differences between the two, too.