Go for the Corian!
While I don't know much about Silestone, I do know a little about Granite and Corian. Granite and Corian are usually very close together in price and they are both recognised as premium materials for countertops, I would vote for the Corian.
I know that some natural stone countertops must be periodically sealed. I do not know if granite falls into this catagorey, but if it does, and it is not sealed as a part of regular care, then the counter can start to become a home fo bacteria and germs. Also, Granite is hard, but brittle. Over time it will get scratched, chipped, dinged, and generally show a patina of age and use. While this is not bad, it may not be to your taste. Also, granite can become stained.
Corian is a synthetic material. It is a bit softer than granite and will show scratches, nicks, and dings more than Granite. Corian has a distinct advantage, however. To make old Corian look like new, have the countertops sanded and buffed, it will renew the surface at a fraction of the cost, and time, of new ones. Corian is less forgiving about heat than Granite, so don't put a boiling pot on the Corian, although it is better than Formica. Corian can be patched and a good installation will result in NO SEAMS. The piece of Corian cut to make the hole for the sink (use an undermount stainless steel sink, not a Corian one) is a good cutting bored, trivet, or heavy serving tray.
Whatever you choose, get a solid countertop, not tile of any kind. While tile looks great, it is a royal pain to clean. The tiles come clean in an instant, but the grout between...
Go for the Corian, my mother got it and loves it. It is super-easy to clean and live with, it looks great, and it will continue to do so for decades. If ever it looks worn and tired, sand, buff, and it's like having new countertops again!
Do not go to a "big-box" store for this purchase. Go to a showroom that specializes in kitchens. They will be the most knowledgable about this kind of thing, and will be able to give you information on the entire product line, as opposed to the lower end or most popular versions.
I hope this has been of some help,
Dave