Tamron or Sigma 18-200mm lenses

I am contemplating one or other of these two lenses which are now both available in the UK and which I intend to use on my istDS. Both are specifically designed for use on digital cameras with APS-C sensors. Has anyone any useful comments on the quality of these lenses or Tamron & Sigma lenses in general which might help me to choose the best one.

There was a head to head review of both lenses in either Practical Photography or Photography Monthly quite recently. From memory there was not much to choose between them but neither of them were all that good.

You might want to compromise to the Sigma 18-125 which has had favourable reviews