GTA3...which I'm waiting for on PC in April, is IMO THE best PS2 game on the market...I'd recommend that one.
Sons of Liberty (MGS2) is a great game in it's own right, looks stunning, plays well. The problem I had with it was that it was a little repetative. Think about this...I never got the sensation I was playing a game, as it seemed it was composed of dozens upon dozens of cutscenes. I'd watch a cutscene, then walk down a hall to find...another cutscene. I mean, come on, there's a cutscene that lasts an hour and fifteen minutes! That's 75 minutes between play time...kinda not good in my book.
GT3 is a great racing game, and it looks good to boot.
It's a matter of personal choice, go with what you like...