I find the ZT60s HD cable picture outstanding, but I also find the F8500's HD very ver good. For SD channels I like the Samsung a bit more, actually. But this is less of a PQ reason and more of a practicality reason, because I love that you can set the F (and I presume all Samsung plasmas) to auto zoom based on the content type. So for my brother, who watches a lot of SD content just out of the laziness on occasion of not finding the proper HD channel on his cable feed, I solved his problem by forcing FULL on zoom on all SD channels so he won't get black bar burn in over time. Can't do this at all with the Panasonics.
The other advantage to the Samsung for day to day watching is IR resistance to the station logos in the corners and news and sports channels. If there's a channel or two that you watch a lot the Pannys will develop IR in those areas, even though they have better PQ in my opinion. Samsung is more resistant to this. Everyone here knows by now I'm a huge fan of the Panasonics and the ZT in particular even though most people don't see the point of the upgrade over the VT. But even still, the Samsung would be my simplest and most honest overall answer to your question based on personal experience with an F4500, VT60, ZT60 and F8500 and their associated behaviors.