Foremost, price is an issue, these are comparable products so the price should be +/- 5 to 10%.
If you're more into SD content - the Panasonic has the edge. The stretch modes are a bit more versatile. The upconverting qualities are comparable.
I am very happy with my Sammy A550's ATSC tuner, fast, fairly sensitive and has an EPG. That's is important to me so I'm not complaining about the SD content even I know it could be better.