I haven't actually got around to playing ICO HD yet, but I'm halfway through SotC HD, and I must admit, though it pains me to say, I'm a little disappointed. Not at the gameplay or anything like that; I played SotC on my PS2, and it's just as amazin now as it was then. I actually feel a little short-changed on the whole HD remaster part. I mean, yeah it looks a lot nicer, and the consistant framerate is nothing to sneeze at, but I still get some degree of texture pop-in, weird shapes that shift and change on the horizon (and definitely not landscape-related), annoying clipping when climbing colossi, and everything still looks a little polygon-y.
Just little things like that which I thought would have been ironed out of something like this. Perhaps in order to fix them, they may have had to rebuild the entire game from scratch, I don't know, but they're just things that I feel could have been done much better.
But yes, buy it. It's still one of the greatest games you'll ever play, hands down.