I don't own both, but I have a Leek and my girlfriend regularly carries a SS Delica, so I'm familiar with both. While they're similar in length and materials, they're very different knives.
The Delica, while no Military or Temperance, "fills the hand" better than the Leek does. Now Ken Onion can design a handle, and the Leek's very comfortable, but it's still a fairly slim knife that gets uncomfrtable for me if I use it for too long. The Delica has less of that problem.
Both knives are tip-up/tip-down reversible.
As was mentioned before, the Speedsafe mechanism, while it falls within the letter of the law, may still not be safe to carry. A balisong doesn't fit the definition of a gravity knife, either, but that doesn't stop police from seizing them or DA's from prosecuting. If you have trouble with a cop for any reason (your fault or not, arrested or not, and whether or not you're charged with anything), there's a halfway decent chance he'll decide you shouldn't have it and take it away. Then you'll have to jump through hoops to get it back (if you even manage to get it back). I dig the Speedsafes, but I'm not sure they're worth the trouble.
I also figure it's only a matter of time before more state governments decide to define these things as automatics, so I wouldn't want to get too attached to carrying one.
Despite the potential legal problems, the assisted opening really _is_ kinda neat. And it's in contrast to the Delica, which'll most likely start out "tight" and gradually wear in (it'll still require a tiny bit of "effort" to open relative to liner locks or axis locks, but I find it trivial. Especially when you're using a round opening hole).
My final note regarding the Speedsafe: beware of cracking springs. I have a three-year-old Chive that's now a manual opener. The "torsion bar" that drives the assisted opening gave out. Of the five Kershaw assisted openers that I've seen in use for long periods, this is the only problem I've seen with a torsion bar. I wouldn't honestly worry too much about it, but it is a consideration.
I have no problem with either lock. standard Spyderco quality on the Delica, and a well-made framelock on the Leek.
The steel is an issue to me. They _seem_ to be roughly equivalent on paper, but I see a noticable difference between Spyderco's AUS-6 and Kershaw's 440A. They're both easy to sharpen, but the Kershaw's gonna need sharpening a bit more often. Not a big deal if you touch up your knives frequently.
I think it's most important to know your needs. If you want a slick, pretty knife that'll impress your friends and serve you well for light use, the Leek's an excellent choice. The Delica, OTOH, is a better alternative for everything from light to heavy use, and it'll serve you well for a lifetime (both knives have very fine points, though, so make sure you use them properly)
The SS Delica is a distinctly better work knife, but the Leek's distinctly "cooler". And neither is what I'd call a bad knife.
On the other hand,
Alton Brown carries a SS Delica, so that kinda tips the coolness balance
