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Spyderco lockbacks are excellent---they are inherently reliable and lock up like a bank vault. The Delica is an excellent knife, a wonderful EDC (Every Day Carry). If you have the funds, I might recommend the G10 model with the fully flat ground blade; if you're looking for something more economical, any model is an excellent choice for everyday chores.
I have a host of different types of locks in my collection, including all that you mentioned---the lockback, the liner lock, the frame lock, and the more innovative locks of our time like the Axis lock, compression lock, etc.
Liner locks are more prevalent, I believe, because they're more economical to design and manufacture. They're a simple lock design. Most of the five-dollar-bin Chinese knives feature these because they're a one-piece design simple to manufacture to bad tolerances. They can be excellent locks, when made to precise tolerances like the locks on the Spyderco Military and such. With all the lockbacks pieces, springs, fittings, etc., it's simply more of a cost to mass produce---that's why liners are very common nowadays.
Get a Delica, or any lockback Spyderco, and you will certainly not regret it.
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