"After a bit of research (!) tells me the minimum fully threaded length of a bolt is 12 + 2xD mm (so an M10 bolt has a reliable threaded length of 32mm) and setscrews cost more than bolts as the thread has to be cut right up to the thread and bolt threads can just be rolled on. Bolt threads are also stronger, as the thread runout is better controlled.
Dull huh? But it's the sort on info (min threaded length) which screwfix should include in the product description.
Hence - use a bolt if you can, they're stronger and cheaper."