Just got a CB Leek recently and have started carrying it. I don't have an Aegis, but I've got the Flash I and II. The Aegis looks like it's a slight bit larger than the Flash II (and different blade shape). Other than the blade shape and length, though, they do seem to have a lot in common: same blade steel, handle material, handle shape, bayonet clip, arc lock, SAT, etc.
So, from my experience with the two (granted, I have very limited direct experience with the Leek so far), here are my observations:
Carrying: The Leek is a significantly smaller knife (thinner, narrower, shorter) than the Flash II, but it does weigh a little more (2.9oz vs. 3.1oz, weighted on my kitchen scale). Despite its bulkier frame, though, the SOG actually carries pretty well. It is deceptively light for its size (probably because of the absence of metal liners), and the bayonet clip puts the entire knife below the top of your pocket (tip-up carry only, reversible for left or right side). The Leek's clip reversible for tip down or up, but it does expose a fair amount of the knife above your pocket (about 3/4" for tip-down or 1 1/8" for tip-up, right-side only). I can't say either has a definitive advantage here, as this is based largely on preference. If you have small pockets, though, the Leek might be better for you. Personally, I like the deep-pocket carry of the SOGs more.
Opening/closing: Kershaw's SpeedSafe by far a smoother and faster opening action, and it's easier to close than the SOG's. The Leek also offers both thumb stud or flipper for deploying the blade, both of which work well. The Flash (and Aegis) is thumb stud only, but it works fine. The Flash opens a bit slower, but that is due to my adjusting the pivot screw to reduce blade play (mentioned below). For closing the blade, the Leek has a pretty consistent level of resistance (from the A/O spring), so closing just requires a steady pushing of the blade back into the handle. On the Flash, once you get the blade to about a 30-degree angle, the resistance increases substantially. It requires a bit more force at that point, and once you overcome it, the blade snaps itself shut (I've nicked my pinky a couple of times because of this). The Kershaw has the decisive advantage in all matters opening and closing.
Locking: The Leek uses a frame lock vs. the Flash's Arc lock. Both locks engage consistently and securely. The Leek locks open very solid, having no blade play at all. The SOG has a bit of horizontal and a miniscule amount of vertical play. The horizontal play can be reduced by tightening the pivot, but that also causes the Flash to open slower. Overtightening the pivot can prevent the A/O from fully opening the knife. To reduce my Flash to an acceptable amount of horizontal play, I had to tighten it enough to where it caused a noticeable slow down in the A/O deployment speed. One benefit to the Arc lock is that it can be disengaged to allow closing of the blade without ever putting your thumb in the path of the closing blade. I'd give the Leek the slight advantage here just because of the blade play on the SOG. I actually like the operations of the Arc lock system more, but functionally, the frame lock does lock better.
Safety: The Leek has a little sliding piece of plastic at the base that blocks the blade from opening until you slide it out of the way. The SOG has a little switch in the middle of the handle that locks the actual A/O mechanism until it is disengaged. I think the SOG safety mechanism has the advantage over the Kershaw. Just in the three days I've carried the Leek, I've already had the safety inadvertently disengage as it brushed up against my keys while I was pulling it out of my pocket (was carrying tip-down at the time). This isn't a problem if you carry the Leek tip-up, as the safety will actually be out of your pocket. Some people don't care for the safety at all and remove it entirely or permanently disable it (eg. using epoxy or super glue), so it depends on what you think about safety locks. Personally, I've never had a knife accidentally open in my pocket, and I'd like to keep it that way.
Blade: The SOG has a thicker and longer blade than the Leek. The SOG uses AUS-8, whereas the Leek has a few options (14C28N, CPM-D2, or S30V). There's no particular advantage for either here, as it depends on your own preference for steel properties, blade shape, blade size, etc.
