I think now that the two TVs are at the same price point, there will be a lot of people trying to make this decision (as am I). I spent an hour at BB the other day, and they had both sets side by side luckily.
The sales guy was cool and we went back and forth with content and light and dark scenes and other use cases.
For 4k content that was bright, and even 1080p movies like the Dark Knight, I found the clarity of the Q8 more to my taste. In the scene on the Dark Knight where the joker is in the kitchen and does the hide the pen trick, the Q8 seemed almost 3D like with the clarity compared to the 900F. The 900F seemed a little muddy or not a crisp. The colors seemed a little over done. I think that is what some people call it having a warm look.
Then on general TV content (we had ESPN up showing a news show and a lot of NFL replays), the 900F looked much better. That better contrast I saw on the Samsung now looked dull because the colors on the 900F were bright and really popped.
For me I think it will come down to figuring out what I will watch most on the TV. If I watched a lot of movies, I think I would opt for the clarity and contrast of the Q8. If I did a lot of gaming I would opt for the q8 as well. I watch mostly TV shows and streaming stuff off Netflix (TV shows more than movies) and some sports. So since it is the case, I will likely pick the Sony.
As many have said, they are so close, it comes down to personal preference and the type of content you will be viewing on the TV. Keep in mind, you won't have the TV's side by side in your house. Just make an informed decision, but it and never look back.