I agree with Ken. I "think" that most state laws will measure the tip to handle dimension. You can also pull up your state regs for concealed weapons blade-length restriction. Most cops don't really know and don't care; they're just "guys." If you are arrested for something nasty and they're looking for something else to charge you with...
When I'm after a 3" blade max, I have purchased an Emerson Micro-Commander, a Spyderco Native 5 Forum knife in S110V (great steel), an SJ75 Strider (super) and a Sage2 titanium. These are all 3" (or a hair less) tip to handle. More correctly, the Strider SJ75 mikes out at 2.980" tip to handle (at blade centerline) and has a sharp edge of 2.820"...the same as the Sng. The Sng has a 3.5" blade-length, with 2 7/8" sharp edge. Note that the SJ75 still gives you a hand-filling grip. I have Large hands. The open length of the SJ is 7 1/8", with a full 4.25" handle. Plus, it has a nice leaf-shaped blade of 1/8" blade-stock, which I really like.
That's enough of that sales-pitch.
Now, for the 561 vs SMF. I've had both; I have two SMF's presently. Blade-steel thickness is 0190"; makes a decent pry-bar but doesn't slice a tomato very well at all. It may have a sharp blade, but a THICK blade is NOT a good slicer, regardless of the name-tag. Striders are tank-tough; so are ZT's.
IIRC, my Sng's were 0.160". I've had quite a few ZT's: including the 560 and 561; excellent knives. The only one I have right now is a gen2 0550 which is a very nice "compromise" folder. It's still heavy-duty, 0.156" thickness and 3.5" length, but isn't such a big ox in the pocket as the 056x series. If interested, I posted a thread including it yesterday..
http://www.bladeforums.com/forums/showthread.php/1118670-So-you-want-an-EDC
I notice that I keep on buying SMF's, but I'm no longer interested in the super-large ZT's. My opinion. Both are great companies with excellent Customer Service.
SMF's carry nicely in the pocket; better...lighter...IMO...than the big 056x series. I'd suggest a CC Strider finish or Lego. Don't do a DGG grip unless you want to tear up your hand AND your pocket.
They're all good. No kiddin'.