I would recommend a high quality stainless in the island, rectangular about 14" by 16" give or take.

I love farmhouse sinks and would install one as the main sink if that works with your style and tastes.

Do you have any inspiration pictures that you can post?

I think it's really personal preference. We have cast iron now, which I love and my husband tolerates. We had stainless before. Pros to cast iron for me are looks (we have a 1915 house so have a bungalow-style kitchen, and cast iron just fit much better than stainless) and ability to hold water temperature (longer than our stainless did, but that may also depend on the gauge of the steel for the stainless sinks--I'm quite sure ours was a cheap one installed by the granite people to sell the house). Pros to the stainless was that it hid grungy corners much better (though of course they were still there!) and took dings better. Back when we did our kitchen, there were more stainless sinks that had workstation style attachments to add; our cast iron is a Kohler that just has a cutting board accessory that covers half of it, and even that has not really been worth having given the price. If you want that type of sink, you'll probably have better options in stainless.

I think either cast iron or stainless will look lovely with your counter. I don't know much about solid surface, so can't speak to that. We have gray and white cabinets with soapstone and wood counters (sink area is soapstone/gray cabs).

I think stainless is the safest bet. We rented while building and that house had a white Kohler enameled(?) sink, not sure what type, but it was a pain to keep looking clean. Anything you put in it would leave gray scratch-like marks, which would only come out with bleach spray and major scrubbing. Was happy to go back to stainless in the new house.

I've used stainless sinks from Julien (Home Refinements) in 2 houses and been very happy with them.

It’s a personal decision. I’ve had both and replaced a crappy stainless steel with a Koehler White Riverby. Love it. I’m just personally sick of stainless anything. I put up with the extra care because it’s pretty.

I love Kohler cast iron sinks--I have had this sink in two houses. I don't like stainless at all. I had a stainless in a vacation house and never liked. It never looks clean and scratched easily.