Cylinder Sharpener

At work we have a cylinder sharpening machine, computer controlled, accurate to hundredths of a mm, has it';s own winch and cost around £25000 reconditioned.
Having used one of the devices you mentioned, I think I'd put my money on our machine
You will only get the cutting cylinder adjusted correctly if it hasn't worn unevenly. You'd be surprised how a cylinder can wear. To get some idea, It can be illuminating to buy a new bottom blade and fit it in. You know the bottom blade is then straight, so you can then see how far out the cylinder is.

Save your £3.50, buy a bottle of beer, it'll be money better spent.