I read somewhere that derringers are for arms length and closer,
including contact shots.
That being the case, it seems a derringer is less effective than a knife.
I'm no expert on the subject, but if you have to get close enough to use a knife to use a derringer, then wouldn't a knife be better since it offers more than two tries?
Those small, folding NAA deringer size revolvers might be another matter. I've heard they can be accurate to 10 or 15 feet and have several shots. That might be equal to a knife and is still approx deringer size.
On the other hand, the Taurus .32 Mag snubby with 5 shots and a hammer bobb job would beat any deringer, NAA, or knife, in my opinion. Likewise the now discontinued S&W .32 Mag Centennials and the like should be awesome too. The only complaint I have with these S&W is they are too light (13.5 oz) and are reputed to kicked like heck. This makes the Taurus number one in my opinion at 17 oz. I think the Taurus would be much better than any knife.
P.S. - I own an SP101 .32 Mag 3" barrel and it is no pocket gun. It does not fit any pants pockets, unless you're dressed like a super baggy pants rapper. It does fit into most of my various coats' right hand pockets, but makes my coat hang way funny and is uncomfortable. Don't kid your self, an SP101 is a great gun, but no way is it a pocket gun.
On the other hand, various models of snubby .38 and .32 revolvers and the Bersas autos, and especially Firestorms autos, and some models of Kahr are great pocket guns. By "pocket gun" I mean coat pocket, although many of these also work well in a slacks pants pocket, especially if it has a watch pocket inside the larger pocket. I find the watch pocket is an ideal holster for the barrel and trigger guard of my Firestorm .32. A Firestorm .380 is the same size and shape and would also work well in my pants pocket.
As an aside, I find that rounded trigger guard guns work much better in my pockets because they are not sharp. The recurve type trigger guards tend to snag when putting into my pocket, wear holes in my pockets, etc. The small shooting advantage of a recurve is not worth the concealment disadvantages in my experience.