Extention tubes will shorten the focal distance not extend the zoom range. So if you are doing close up work and find that you cant get close enough to your subject then tubes are your thing, If you however are finding that you need to focus on a subject that is further away and its just not filling the frame, then a convertor will probably help here.
They are both very different.
I said I didn't use Kenko tubes but thats just because I use a set made by Jessops (a uk camera retailer) as they are just metal rings the quality of picture is not affected so I wasn't too concerned about branding. When I needed them my loacl shop had a set in so I just called and picked them up. I also use a Kenko pro 300 dg 1.4x convertor, this does have glass that could effect the quality but as it is made by Hoya to very strict standards I find the results to be excellent.
There are many examples of pictures I have taken with both these combinations on my 20D available in my gallery, the water drops where done using the extention tubes and the F1 stuff (some of it) was with a 70-200L and the Kenko 1.4x convertor.
My gallery can be found
here
Hope that helps but if not ask away and I will do my best to answer.
