If I had to choose only one, I would go for the multi-tool first (I carry a Leatherman Wave), over a more traditional SAK.
In this day and age, when camping, outdoors etc, you're more likely to be turning your knife/tools on your own gear to fix and adjust things, than you are on "the wild". And then for "taming the jungle", there's usually better tools, camp shovel, pack saw, a hatchet etc. that are better than anything you can carry in your pocket.
However, most of us here love our gear, and are always looking for excuses to get more, and would ultimately advocate carrying BOTH!
The main downside from the blade standpoint of Multi-tools is they often have the blade fixed into one of the handles at a funny angle, or a weird offset all the way to one side of the tool selection near the edge. Compared to a dedicated knife, which is proportionally not as thick to grip, and has the blade aligned better with the centerline. (Although the monster "kitchen skink" SAK's have the exact same problem as multi-tools.), so for fine cutting or carving/whittling, a SAK or other more "dedicated knife" is better than most any multi-tool IMO.
Frankly, I could almost justify carrying everything below all at once, were I not afraid of looking like a geek or a "nut": (And this is just knives and tools)
- TWO multi-tools, so I can use the pliers and other tools in opposition like you would on bolts/screws. Also to maximize the variety of tools usually not available on any one model alone. (What good is the pliers on a rusty nut if you don't have a screwdriver or second pliers for the bolt?)
- "Super knife", This is the utility razor type knife with replaceable drywall blades for when a razor is better than a thicker knife. What makes it "super" is that it folds and looks like a pocket knife. Good for careful cutting, peeling, or cutting lots of cardboard, where you don't want to dull a nicer knife you spent time sharpening.
- A large "tactical folder" for, well, umm... I guess things and situations that are "tactical".
Or for when you just need a bigger blade.
- A medium/small SAK, Case, Buck, or other "regular" medium pocket knife for whittling, or general utility when all the other blades just don't seem "right" for the task.
- A small "gentleman's knife" like the Kershaw Chive, one of the cuter looking Spyderco's, or the ubiquitous mini-SAK, for use around "Sheeple" (Overly nervous hyper-PC types who treat any blade you've got on you like it's a mugger's switchblade, etc.) Sometimes the Multi-tool fits the bill with these people, because their addled brains are able to view it as a "tool" first, because of the pliers etc.
- A keychain blade of some kind, so you always have something with you at all times. Like the mini-SAK, one of the tiny Spyderco's, or maybe the Gerber "Ridge". (Open faced knock-off of the mini-CKRT K.I.S.S.) Maybe a Leatherman Micra.
- For true woods outings, I'd wear a larger sheath knife as well. For campground camping, car-camping, etc., on "improved campsites" with pre-made fire pits and outhouses, I wouldn't bother.
Of course, I wouldn't carry all that, even though I can justify each of them in my own mind. I might carry all that if I could do so without getting "caught", in this case meaning being derided as "Batman" or "MacGyver" by my family and friends.
In reality, I carry a mix of the following, depending on the day and the situation. The Leatherman Wave as my multi-tool. An Emerson/Benchmade CQC-7 as my "large tactical folder", and as just the "big blade" for cutting "big stuff". And a Gerber Ridge or a Kershaw Chive as the "gentleman's knife" or "small PC blade". I bring the folding replaceable utility razor "Super Knife" along when I think I'll be doing some kind of work, or a lot of box cutting I don't want to dull my other blades with.
Essentially different situations require different tools, that's why there's so many to choose from!