Many people ask what size hook they should select for their Mop Flies. Size 10 or Size 12 hooks seem to be the most popular sizes in Fly Shops and online. There are a couple of variables associated with each size. A 3.8mm (5/32") slotted tungsten bead is typically used with a Size 10 jig hook, and a 3.3mm (1/8") slotted tungsten bead is typically used with a Size 12 jig hook. Obviously, the Size 10 hook is slightly larger than the Size 12 hook, so the hook gap is slightly wider on the Size 10.
So what difference does it make? The Mop Fly body tied on a Size 10 hook is essentially the same size as the body tied on a Size 12 hook, so from the fish's perspective, the profile of the fly is basically the same. But the Size 10 hook will be more heavily weighted (because of the 3.8mm bead) and get down to the bottom quicker and stay there easier. The Size 12 Mop Fly would drift a little higher in the water column, and that may be advantageous in some situations.
As for hook setting (and holding) advantages, the Size 10 is probably a little better because of its slightly larger size. But some anglers think that the Size 12 is better for smaller fish (although arguably, if a fish can get a Mop Fly in its mouth, it will likely be able to take a Size 10 hook also).
At the end of the day, the difference in the weight of the associated tungsten beads is probably the biggest difference between using a Size 10 and Size 12 hook. There is a place for both hook sizes in every fly box.