I have had both projectors. I currently have two UB6500's. I keep a spare in the closet just in case.
The 8350 is a really nice projector and a great projector for the money. However the 6500 has better black levels.
If you had not already been using a 6500 you would probabley not notice the difference and be happy with the 8350.
Now that you have had the 6500 you may notice the blacks a little greyer? Not quite as good but most people never even notice.
My best friend wanted a home theatre like mine but did not want to spent the money I did so he bought the 8350. He uses it everyday like a TV. He is very happy with it. It's not bad. But, when I watched it at his home I noticed the blacks were not as good as my 6500. He still had a very good picture.