Looking at Cnet's sample images, in my opinion the WX150 is a bit better. You give up the long zoom though; the Sony only has an equivalent 250mm telephoto lens, while the Panasonic goes to an equivalent 480mm. The ZS20 is a bit wider too. The Sony has an edge in video too, recording at 60fps vs. 30fps on the Panasonic.
I was almost was ready to get the ZS20. But then I noticed the Sony DSC-WX150, which had some good reviews. I would say that I am still leaning ZS20. Which one out of the two do you think has better pictures and which one has better video?
The DSC-WX150 is $50 cheaper than the ZS20, but it doesn't have the GPS...which as I posted earlier isn't an issue with me.

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