Several folks have correctly asked about intended uses, and to be honest I really don't have a great answer. When things go well, I'll use a knife to cut an apple. I only do day hikes, but lately we've been in situations where my wife and I felt like there's no excuse for not being better prepared. We always have food and water, but I'd like to be prepared if I twist an ankle, slide down a slope, etc. and have to walk back in the dark, spend a night on the trail, or even wait for rescue. So I'm looking at adding things like a knife, headlamp, fire starter, water filtration/treatment, thermal blanket, etc. to the day pack.
When it comes to tools like a knife or bear spray, my thinking is that it should be readily available. So that's why I'm looking at a fixed blade that can attach to the pack's front strap, versus just throwing a folding knife or multi-tool in the backpack. But again, my hope is I'll just be using it to slice apples, etc.
The question you really need to answer is "what do I need the knife for".
The major difference between a folding knife and a fixed blade knife is two-fold: The folding knife is it's own sheath and takes up less space lengthwise. The fixed blade is a stronger knife, not having that folding joint.
That's it.
Now you need to answer the question above. Do you need the knife for hacking, stabbing, slicing, prying, chiseling, skinning/scraping, filleting, scaling, etc.?
There are a great many blade designs out there and while it's safe to say they all perform many of the same things as any other, they are not all equal in their capabilities to perform a particular function/functions.
A filleting knife WILL cut, slice, stab, etc. But whereas it will do all these things, what it's really designed to do well is fillet a fish. It most certainly does not have the strength to do much prying and is way too straight/flexible to perform well at skinning/scraping. Likewise, it would not be the best choice as a defensive weapon.
In general, if you want just a decent all around survival knife that will perform several functions pretty well, what you want is a full tang knife (strength), decent blade length and shape that will allow a variety of cutting functions to be served, with a thickness that will enable to withstand a decent amount of hacking or prying functions. It should be of a decent steel that will hold an edge requiring no more than a common sharpening stone or similar device to maintain. It should have a quality sheath that will both keep the knife safely secured and allow ease of access when needed.
There are many knifes which will fit this role. And, quite frankly, there is also plenty of room for personal aesthetics in the matter. Prices range from affordable to OMG!
And keep in mind that, like firearms, there's no reason why you cannot have more than one for any reason. I'm all about choices. Carry a decent pocket knife as well. Have a cool-tool (Leatherman like tool) in your kit, too. Have several knives at home you can choose from based on your anticipated upcoming hiking needs. Going fishing? Make sure you have a scaler/fillet knife. Gonna be in brush? Maybe a longer, heavier knife should come along on the trip. Camping in the rough? Something capable of serving up some fuel for the campfire.
