I only clip the front claws because those are the ones they tend to do damage with. Every once in awhile, the back ones are clipped because hubby complains about them digging the claws in when they launch off his lap.
I prefer the scissor type because I can control them better. I don't think I can get the right angle with the guillotine style. The claws don't get any tougher as they get older. I use the same ones on Much who is 12 as I do on Carly and Lucy who are around 3. And these are the same ones that I used on Lucy as a Kitten.
I do recommend replacing them every few years because they do dull over time.