I personally like a mix of knives. I usually have at least two. A bigger blade, which is mounted to my pack or in my vehicle if I do not carry it.
A smaller blade, 5-6'', on my person. Occasionally, I will carry a 4" blade such as the benchmark Ninja, depending on situation and what's going on.
For survival? The scrapper 6 is a perfect size to carry. not too big, not too small.
Other choices? Reeves aviator 4" or 5" is a good bet. The new Busse badger attack might also make an ideal carry blade, in a survival situation.
Right now, I carry the scrapper 6 almost constantly. It is mounted in the factory sheath to the molle straps on my eagle pack.