I will do a 17" round prep sink in my kitchen when we redo it. You will probably get many answers going against it, but I don't need it to be huge. I love the look and my main sink will be close to it.

I would match the prep sink finish to the main sink, either shape. And it would be stainless on both. I am sure your appliances may be stainless or would hope they are. That's a forever finish.

I'd prefer 15"-18" square/rectangle. Is this the primary prep zone? In a 5' island you need to consider the available space--minimum 36" on one side for efficient prep space (NKBA guidelines/see #12), which will leave only 6" on the other side, if you use an 18" sink. Starcraft, in the31 Design Rules, has changed that to 30", so if you follow that guideline, you'd have 12" on the other side. Note--if the cabinetry alone is 5', then the top will have overhangs, which will net you another 1-1 1/2" on each side of the sink.

It's nice to have enough counter to lay un-rinsed produce, which leaves the prep side clear for the washed items, but I've seen prep sinks with only a couple inches of counter on one side. My main sink has only 9" on the right side, next to an open space, which works OK. We are adaptable.

Rectangle. Think about how well a small round, shallow sink will help you in prep and whether or not every drop would splash out onto your countertop.

Thank you, everyone. As it turns out, the decision was made for me when I looked further into my cabinet order. The round sink requires a 19" base, and mine is only 18". Rectangle it is!