Sigma 24-70EX or Tamron 28-75??

I need a walkaround lens for an upcoming holiday and, with the Australian dollar sinking disastrously, my initial choice of the Canon 24-105L has had to be shelved. Of the above two, which would members pick as having the best IQ and which would overall be best for a UK/France holiday. I've seen some great photos from the Sigma, but I believe it's a hefty lens.
Any other suggestions that would comfortably outperform my previous lens, a 28-135IS?
I still also own a 17-40L , great lens but woefully short as a walkaround, and a Sigma 70-300APO. Between my camera, my wifes camera and our video camera though space, and weight, are big factors. Has anyone else used the 17-40L as they're only lens on holiday? Will mainly be shooting the usual UK etc suspects, castles, streetscapes, landscapes and the Chelsea flower show.

Thanks,

Hal55

Crop or full-frame camera?

I've owned both. I think the Tamron might focus a hair faster and might be a tiny bit (hardly perceptible) sharper, as well as lighter. I prefer the contrast on the Sigma though.

Sorry, should have specified that, either an XTI or, preferred, an xsi.

Hal55