I think it really depends on what the story and setting in the game are going to be. In Twilight Princess the legend was set in a drab, slightly more industrial Hyrule. There were dark forests, empty plains, and frozen mountains. That's not a lot of colour. Coupled with the fact that Twilight encompassed many areas of the game for a good chunk of it, and the fact that so many bad things were happening to people, moral was not very high. I'd say that the main colours of Twilight Princess would be shades of brown, orange, dark green, and gray. This is a huge contrast to what we've seen so far of Skyward Sword, which features bright cheery colours, similar to that of Wind Waker. The graphics in TP were also a lot more "harsh", yet at the same time they were blurry and washed away. Again, this has a lot to do with the colours more than anything. However there is also the fact that Twilight Princess doesn't use the same style of Cel-Shading as Skyward Sword. TP focuses a lot less on shadow and contrast and looks more toward the drab and blurry end of the spectrum.
Skyward Sword, as I have said, features rather bright and cheerful colours, at least in the area that we saw in the E3 Demo. There is high contrast and everything is clear cut. However, this is not enough to go on. We have no idea what the overall atmosphere of the game is going to be like. Maybe just the forest area that was featured is "happy" and colourful. Maybe the rest of Hyrule is a bleak and empty wasteland, more akin to Hyrule in Twilight Princess. Further more, we have little idea of what the story is going to be like and how it will be played out. I simply do not know enough about Skyward Sword to make a call on graphics in that sense.
However, from a more technical standpoint, considering the fact that Twilight Princess was and is a GameCube title, and was designed with the GameCube in mind, it is quite impressive. Even on the Wii the hard, realistic visuals in Twilight Princess hold up rather well against other realistic games on the Wii, regardless of the fact that the games on the Wii were designed with the Wii in mind and would have a fair amount more power to work with in the graphics department. I am really unimpressed by Skyward Sword in the technical department. The textures are rather sloppy and boring. They also have little detail. The models in the game, at least for the background and enemies, was really unimpressive to say the least. However, I am hoping that this is just because the game isn't finished yet and they will polish up everything nicely for the release in the coming year. I will stay hopeful. So in this category, TP wins.
Overall what I am trying to get at is that there are different types of graphics. Pure, raw technical power, and the style of the game. I can't determine who wins in the style department because I don't know enough about Skyward Sword. However, in the tech department TP wins... So far anyway. I will wait till the game is released to make my final verdict.