I've used both types of flaring tools for brake lines both ISO and double flare.
One has to be stuck in a vice and you have to crank the handle as hard as you can to flare it. Both times I tried, I was able to flare a line, but it didn't come out real nice.
I also used a hydraulic tool where you simply press a lever to make the flare and it's done. Both of my attempts on that tool came out really nice.
Since the difference in prices are so great, and if you get enough practice on the cheap one, I may go with the cheaper one and try to get it right if I'm only flaring lines one time. But if I was doing this working for a shop, the hydraulic one is definitely worth the money. Also the hydraulic tool is really a 2-in-one because it can make both double and iso flares. There are two different types of the manual ones for double and ISO.
