Abs,
You can't go wrong with either choice, but if you like the Onkyo sound then I would recommend sticking with Onkyo. I would also suggest going for the 3008, which means you will have around £500 (ish) to spend on something else. I used to have the Kef TDM's myself and I don't believe they would benefit from difference that the 5008 offers over the 3008. I had a short time with the 906 and never really liked it, but the 3008 is a huge improvement, which believe is mainly down to XT32 being far better.
Thanks for the reply, in the end I went and ordered the 5008 which I bagged for a very good price. Should be with me by end of week or early next week.