I have had both stainless and porcelain. I have preferred stainless. The scratches do not bother me. The texture of the composite sinks is very unpleasant to me, and the sound of things scraping against them bothers me. In the interest of full disclosure, I may be an outlier with respect to this opinion.

Corian! Does not show water spots in hard water areas, is easy to clean, and wears like iron. My KD says it’s the best sink out there! I’ve had mine for 16+ years and it looks like new.

DD put one in her house, too. Her DH is a SAHD and makes french press coffee all day, forgets it, dumps the grounds in the sink and leaves them there - multiple times all day. Stains come out wit a swish of detergent.

Stainless

I have had stainless. I now have granite composite for my prep sink and porcelain on cast iron for my clean up sink, both of them in white. The stainless itself was fine but with my hard water I was constantly battling water spots, which got old in a hurry. So for me the granite and porcelain are easier to maintain - or at least, if there are water spots they can’t be seen. Either way - stainless or other material - I think your single bowl, undermount choice is definitely the way to go! I don’t think I will ever go back to a top mount (drop in) double bowl sink!

Stainless, but be aware that not all stainless steel sinks are created equal. Get a very good quality for looks, durability and strength. I love Franke sinks--they are a bit pricey, but worth every penny.

Cast iron. Stainless is fine from a purely functional point of view, but a cast iron sink has a solidity about it that's hard to beat. Nothing tinny about the way it sounds as you use it.

Good quality stainless, I am a clean freak and honestly have never had water spots that make me crazy and I run a catering biz from my house. I had cast Iron sink in one place we rented it was horrible chipped and discolored I am sure the new ones are better but I always go stainless.

I have to use a diamond blade in my saw or grinder when I have to trim quartz sinks, that's how hard the material is. It is difficult to scratch, but if you do I'm not aware of any cost effective method of removing the scratches. I've sold over 50 Elkay Quartz Luxe sinks and have yet to hear a customer regret.

Stainless scratches, but it is easily refinished.

Corian sinks are tough and beautiful and can be refinished.

One dropped pan one time on cast iron or fireclay and you'll stare at that chip until you replace the sink.

Stainless. The workhorse of kitchen sinks. They can take about any kind of abuse and still not leak.

Stainless.

If you want to keep your sink looking new, I would recommend getting the stainless racks that fit the bottom of the basin. Some don't like them but I have them in two stainless sinks and a cast iron sink and wouldn't go without them. When using a colander it also helps with keeping backwash out of the colander. They easily pop into the DW.

For our new build, we purchased a black granite composite sink, which I love.

In our previous house had a stainless sink, which after numerous years showed the scratches

I think this is a personal decision once you are looking at a quality sink. I love my cast iron Whitehaven. I had cast iron before and never had a chip, but I know it can happen. I have the the grills for the bottom and they prevent a lot of scratching. I have disliked the looks and water spotting of every stainless sink I had back in my apartment living days. However, as can be seen from the responses, some people love stainless and other materials too.