They both operate on the same principle--conduct the heat of the water through the cast iron into the air. You're trading aesthetics off. One issue is that you need a fairly long run of baseboard to give the equivalent heating "power" (i.e., amount of heat given off by a unit) to equal one of those decent sized old radiators. A heating guy can do the calcs, but when we did a renovation we mostly reused the old radiators in part because we needed a lot of wall if we wanted to switch to baseboard.

I had a Kohler, navy blue cast iron sink for many years and it was great. The finish became dull in some places but I could always give it a good scrubbing and then rub a little olive oil on it to restore the shine. I just replaced it with a fire clay white sink and I'm still somewhat afraid of it. The porcelain coating on the cast iron could chip, but I'm afraid the clay sink could actually break. They are both heavy. Delivery and installation was a tense time. Kohler's website has cleaning instructions for both kinds. According to them- the clay seems easier to clean. My friend, a carpenter, said he's been seeing a lot of clay sinks in new installations lately. These decisions seem so important and un-important at the same time don't they? I'm telling myself that eventually I'll get used to whatever I wind up doing.

I've got the Kohler cast iron (and love it.) I'm sure if you dropped a glass in either the Kohler or the fireclay, the glass would likely break. I doubt there is an appreciable difference between the two. I haven't tried dropping a cast iron pan in my sink, but I did beat a Kohler display sink with a SS frying pan-no chips or scratches-though a non-enameled cast iron surface could possible scratch (just guessing here.)

I've commented quite a bit on my soapstone counters in the last few months. There is a lot of information on this board - it helped me tremendously. I really appreciated this review before making a decision too. http://fortheloveofahouse.blogspot.com/2014/03/soapstone.html?m=1. And I commented on this thread with review and some pictures: http://ths.gardenweb.com/discussions/3453446/soapstone-and-the-dish-queue. I also have a fireclay sink and like it but it worries me, hopefully unfounded. I don't have a garbage disposal which I believe can cause craze. I use a sink grid because I am afraid of scratching or dropping something heavy. I am careful as I bring a heavy pot to the sink because I feel like the front is delicate. Funny that I am less worried about hitting the edge of ss counters- that hasn't happened yet. Just last night I tried Bee's oil wax for the first time and like it better than mineral oil. It feels nicer to the touch and I've read has more staying power. The counters I would choose again in a heartbeat but I regret I let our contractor talk me out of soapstone sink. We have "stormy black" harder than many and dark but with some nice veins.