which is better a corian sink or metal on a granite kitchen counter ???

Granite is better, such kind of countertops can last for a very long time and they are easy to clean, this is why many people go for them, they are also available in various designs and colors.

I side, heavily, with Stainless Steel. I've seen and used 50-year old ones that still look like new, restaurant kitchens are loaded with Stainless. They can be sanded and brushed right back to new for, at least, another 50-years of use. If you're silly enough to use steel wool pads and especially around a Stainless Steel sink, you can renew it by rubbing the shiny side of aluminum foil on the I-won't-do-that-again rust spots to remove the problem permanently.

- The very few Corian's I've seen look pretty nasty with stains and gashes. Yes, they can be sanded and even ground-down, but no-one does that since you have to get involved with 4 or 5 types of sandpaper and polishes to get the sink back to respectable.