Well, to be honest, we chose our countertop company because it was the only one in town that carried enough soapstone for us to choose between different slabs. Other companies could order it, but didn't have multiple slabs available for us to look at in person and choose a veining pattern and tone we liked. Everyone had granite slabs, of course.
We also asked our contractor and architect whether they particularly recommended one fabricator over another or if they'd had bad experiences working with any.

So, in short:
1. Wide selection.
2. Reputation.
That's it.

Oh, and they let us take home a good sized sample chunk of soapstone and try scratching it and abusing it in any way we could imagine, then oiling it, waxing it, and generally seeing how it held up and what different finishes looked like. Very helpful.

1. Warranty/Service
2. Value
3. Quality Craftsmanship
4. Selection
5. Reputation

Also please no free stainless steel sinks that will be rusted and leaking after a short time! Within the industry a lot of granite shops will purchase a no-name stainless sink for $30-50 wholesale and "throw-it-in". Now if the typical granite installation is $6-15k the last thing you want is a low quality sink.

http://www.poshhaus.com/

I think reputation drives it all and will give you an indication of all of the things mentioned in the well made comments above.

Thank you all for the input! Here at Florida Custom Marble we have been in business for almost 40 years and our reputation is really good. We do not try and compete with the "Free Sink" stone shops rather we offer Great Service, Quality Products in the the proper amount of time to achieve such a result and at a competitive price but not the "Cheapest". We do not argue that another company knows what their product is worth. You really do get what you pay for. Thanks Again!