Frame lock if you want fast one-handed opening and closing; back lock if you want a lock you can operate identically with either hand and a design with a very long track record. Both are strong enough for normal cutting. The real difference is how they feel to use, not how much they hold.
A frame lock cuts a section of the handle slab itself into a spring that moves behind the blade tang when open. A back lock, also called a lockback, uses a spring-loaded bar along the spine that drops into a notch in the tang, released by pressing a cut-out partway down the spine.
See lock bar for what to look for in a frame lock.
A frame lock closes one-handed with a thumb, which is quick but puts your fingers near the closing blade. A back lock usually needs two hands and is genuinely ambidextrous, because the release sits on the centerline.
Back locks tend to be very secure feeling and are the traditional choice on hunting folders. The Buck 110 is the archetype.
On a frame lock, where the lock bar sits when the knife is new. Early engagement, a quarter to halfway across the tang, leaves room for wear. Late engagement on a new knife does not.
On a back lock, that the blade does not move at all when locked and that the release is not so stiff you cannot work it comfortably.