I love Splinter Cell!!!
But I heard Thief was scaaary like none other! And as you can read above - I don't do scary games... I don't do scary anything!!!!
Same here - I'm a HUGE fan of the Splinter Cell series.
Thief is scary, but only in a couple of levels, including the one I mentioned. But the rest of the game is awesome, so you should try it sometime. You'll love it.
BTW, what game on the Xbox do you think is the hardest? I played Ninja Gaiden, which is probably one of the best and hardest games to play. Like Thief, it's also got a pretty long campaign, so try that out if you haven't.