Splinter Cell, all the way. The best part of that game was figuring out how to get around difficult scenarios, it is much more of a thought-intensive game than MGS2. You're one lightly armed guy, trying to pick your way through a room full of guards. The intensity level is really unmatched by pretty much anything else on the market. After I beat the game, I looked forward to the Live expansion levels like nothing else.
Speaking of MGS2, I got about 3/4s of the way through the game before I threw in the towel. It was BORING. Don't get me wrong, the parts you actually played were pretty decent, but I was so sick of cutscenes I pitched the game on the shelf and never felt the urge to break it out again. I loved the original Metal Gear on the NES, BTW.
I turned 30 last week, so it's possible that MGS is for the younger crowd, and SC is for the "mature gamer". I have a 21 YO and 25 YO working for me, and neither of them really liked Splinter Cell all that much. Basically, it's not "run'n'gun" enough for them.
That said, I haven't yet bought Pandora Tomorrow. I'm STILL waiting for a cheap-ass deal! What's the story here? I have $73 worth of Best Buy certs, but I'm waiting for it to go on sale. I hate paying $50 for a game, regardless of how badly I want it. The fact that I'm still playing Ninja Gaiden has something to do with this as well (on chapter 12, so I'm getting close to the end).