I like the "velvet look" of 400 grit on both Soapstone and hardwood tops with oil. It's all personal preference, there is no right or wrong look.

Well, I have put in my own counters in my kitchen 1-1/2 years ago and I don't think I went above 120 or so. But mine is softer stone and I like the fuzzier feel of the coarser finish. I just did a soapstone sink for my bathroom and I finished it at 120 and won't finish any high than that because it too is a soft stone and will scratch easier than yours. That is my rational for staying courser. I am still working on the counter and backsplash and I have it at an 80 right now and I like that too. 400 will be very smooth. It is very good that you have samples to compare. Do what you like and It will be beautiful.

The granite fabricator is probably talking diamond polishing pads, not sand paper grit. 400 pad is much better than 800 pad in my opinion. 800 leaves a polish, 400 cuts it down some.

You are right. He was talking diamond polishing pads. Should it be less than 400 though? I am hoping to really avoid water marks and such and I think the finish level factors into that.

It really depends on the hardness of any one stone, (or wood for that matter). 320 on one stone may have the same results as 400 on another. Easy to trial and error on a sample piece of your stone, maybe the sink cut out...