Corian is a good product, but be wary of similar yet cheaper versions of it. Some do scratch easily. Granite or any stone can be chipped. You can get filler to cover over it, but it's stone.. it's forever. Install isn't too bad on them unless the walls are crooked, but it's no different than installing other rigid types. Another benefit of Corian is that it can be custom shaped to fit a space in any size, and it is seamless. The measure is done with a laser, really accurate. Granite and other stones are typically measured the same way. Or should be, once it's cut, it's cut. Prefab stone works if your walls are straight and even. If there is a variance, it will show and will need custom fitted with trim to cover it up. Laser measured stone is cut to fit the space. Usually things are even enough to go smoothly.
I worked at a granite installer for a spell, you can get a better deal if you go to the source. The one I worked at did not do business with the public directly, but some do. Choose the company carefully though... the counter tops are only as good as the install.
It won't hurt to ask the kitchen designer at Lowes about a free sink upgrade or edge style upgrade to sweeten the deal. They'll haggle for big jobs like that. Well, some stores will. Just let them know you've been shopping around for installs. They come with a 1 year install warranty, and are a big enough company to hold to it. Some places may be cheaper, but choose carefully.
I like the look of tile counter tops, but I don't want all that grout to clean!