FBJ - I was in your position a couple of months ago, i wanted my hair straightened but i didn't know whether to go with Yuko or Ionic straightening.
In the end, i went here.
A stylist called Lisa did my hair and she was brilliant. my hair is so soft and silky now. My hair is short (jaw length) but I am trying to grow it long now as I've never had long hair before as it would grow into a big bushy triangle
I will definitely go back in a couple of months. She recommends at least 4 months before retouching. I can see the kinks starting to come through, especially a stubborn on right at the front but I love it.
Like you, I've got some grey too and in an ideal world, I would like to be able to go a bit lighter but Lisa said "No!" so I've just been using an all over permanent colour same as my own dark brown hair.
BTW, my friend has had the Yuko system and she really likes it too. she goes somewhere in Mayfair. The reason I chose Ionic over Yuko was I was led to believe you could lighten the hair afterwards but it seems no hairdresser wants to risk it!
Good luck with whatever you decide. From talking to Lisa, she didn't think there was an awful lot of difference in the 2 systems.
Sorry to go on! bit long!