Sorry, I've never even heard of the JCB brand so I can't help you there.
Re lengths, as a very general rule, I find "the longer the better" works for me, but in reality, as you'll see from the photo of my files storage above, I have a good mix, including the odd one or two at 4 inches, quite a few 6 inches, a few 8 inches, some 10 inches, and quite a few 12 and 14 inches. An awful lot depends on what work you're going to be doing, but basically, I think you can't have too many files if you do a lot of metal fabrication and don't have a milling machine.
And don't forget the "little" Swiss and warding files too.
HTH
AES