Neither is the second coming of god (or X time depending on your beliefs)
IMO... Ico itself it was one of the better ps2 games when it was released which wasn't saying a whole lot if I remember. It was also a game that tried to go in a different direction than other games. Which is good if it entertains you.
And while the game was great. I don't particularly remember it more fondly than many other PS2 games that came later. The game relies on atmosphere. And the gameplay for me, was clunky, interesting at times and bland at others... kinda like silent hill 2... 3 etc imo. Only in my case I thought the atmosphere/story in Silent Hill 2 was a lot better and interesting (Yes I know different type of atmosphere but atmosphere is atmosphere)
Shadows of the Collosus is a much better game imo. Because the gameplay was great. the boss fights are awesome the atmosphere is great. However... there's a lot of roaming searching for the collossi on your horse... some people loved this. I loved it for a while, but then I found it bland after a few hours of playing it as theres nothing else to do. I just wanted to beat the bosses which is mostly what there is to do and that's what I did. But my opinion has to do with... some weird twitch I had bugging me... saying, make zelda like this. As in put the in between zelda stuff and then have SoTC type boss fights but thats another topic.
Also, the slowdown is aweful. Slowdown really bugs me much more than polygons even if the artstyle is good (as it is in SoTC). However I played it on PC emulator and it upped the resolution and framerate to perfect. So I still recommend it even if you didn't think ico was all that great Particularly if you have a powerful enough computer.