I've googled this issue a couple of times, and from what I can tell, it's a mystery. You can even go to the website of some of the credit agencies and look in some of their Q&A areas, and they just talk around the issue without answering it.
I talked to my broker again, and he now is indicating that there's really not much of a difference on your credit score, BUT that with a foreclosure on your record it precludes you from getting a "conventional" loan for four years. He said that with a shortsale, it's probably more like 2-3 years before a lender would give you a conventional loan.
As for how long they stay on your credit report, I'm pretty sure a short sale stays on for a full seven years, but that it won't preclude you from getting a new conventional mortgage after 2-3 years. I believe that is what most RE agents and brokers mean when they say it's only on your credit for 2-3 years.
I'm not sure about the "forgiving tax", as I've never heard of it. But I did talk to my CPA about my situation and the different tax consequences for a foreclosure vs short sale. He indicated that there was no difference. But I suggest everyone talk to their own tax person.