Both of the folding knives I discuss here were gifts and for both I am grateful and appreciative. But I can't help but contrast the two, very different, knives regarding some convenient qualities of a folding knife who's job is secondarily as a self-defense tool. The points I'll make below are largely based on qualities I learned to look for in InSights Training's Defensive Folding Knife course, although they've been echoed and reinforced by courses taught by a Martial Blade Concepts affiliate and Tactical Arts Academy seminars.
The two knives in question are:
My everyday carry cutting tool and "jujitsu repellent" is a Spyderco Delica, was a gift for my birthday in 1999 (so it's not exactly the same as the pictured knife, but very close). Notice the mid-spine lock and hole in the blade for thumb-guided opening.
My new dress knife, a present for my birthday in 2018, is a Zero Tolerance Sinkevich 0450. This is a beautifully finished, smooth-operating, aesthetically pleasing knife. Notice the frame-lock and "flipper" opener.
In my closed Sinkevich, very little of the blade is exposed from the handle, and it's very smooth, so I can't reliably grip it with the fingers of my support hand. That is, I can't use my support hand to open the blade in a two-handed draw-stroke. That's the method I always prefer for folding knives because it's surest and fastest, and the Delica supports that method well.
The Sinkevich is difficult to open surreptitiously with one hand. I have to move very carefully to open the blade part-way, then (dangerously) use a finger to mute the lock action, then finish the blade opening with little or no purchase on the side of the blade for friction. It's easier to do two-handed. But with the Delica it's easier still in both cases.
There's little tactile feedback that the blade has opened all the way and locked in place. With the Delica, I guide the blade all the way into the open position. If the blade stops opening for any reason (e.g., it bumps into something) I know it immediately because my thumb stops, too. If you've ever tried opening an inert folding knife during grappling, you've experienced a fouled blade-opening and understand why this is a vital piece of information.
I can confirm that the Sinkevich blade is open by executing the flip, then moving my index finger up along the spine of the blade, or thumb up along the flat of the blade, to feel that the blade is open. But this is a significant slow-down, which seems to defeat the very purpose of the fast "flipper" mechanism.
The grip I need to activate the Sinkevich is farther from the final grip I want than it is with the Delica. With the Sinkevich, I start in nearly a "scalpel grip" (index finger along the grip spine), and I must reorient the knife in my hand more than I do when I'm rotating the blade open with my thumb pad. That said, I have almost 20 years of practice opening the Delica, and this point will vary substantially based on hand size, shape, and mobility.
Frame-Lock + Flipper
In the Sinkevich, the combination of a frame lock with a "flipper" to open the knife has an interesting consequence: a very firm grip on the knife makes activating the "flipper" difficult or impossible, depending on the strength of grip and position of your fingers. I think it's more likely I'd over-grip the Sinkevich because its (beautiful) smooth titanium grip/frame scales are without texture or tackiness. I also suspect it's a challenge to hold on to when wet.
Reverse Grip Opening
I prefer a tip-up, blade-out grip, but I know a little about employing it in a tip-down, blade-out grip, so I might as well comment on opening the knife in that orientation. I can open the Delica one-handed in this mode using my ring finger to guide the blade open. Perhaps there's a great technique for a one-handed reverse-grip opening with the Sinkevich and other "flippers" but I haven't figured it out yet. I can make it happen using my pinky finger to flip the lever, but my grip feels tenuous and my pinky strength and dexterity isn't in the same class as my ring finger.