I have some of both. The push pin locks are easier to manipulate than the slotted locks, especially if you put them on upside down.
With some cuffs, the push pin is on the outside of the cuff, so there is no "upside down".
The double locks " lock" the cuff from getting any tighter. If you dont lock it, every time the cuffed perp moves, it will get tighter and they will start complaining about it. If you double lock it, it prevents that.