I use the Knipex 'dismantling tool' (catchy name!), and it is excellent. The time saved when dealing with flex is fantastic. And no more risk of slicing my fingertip off, which is nice!!
I went for the one that costs around £25 I think, have tried it with different sized flex, and also with some T&E, great results all around. Obviously need to check insulation afterwards for any nicks, so far so good.
EDIT: I just saw that you were looking at flat cable strippers. If you hit the flex with a mallet a few times it will be flat.... ;-/