In October and November I've seen 42" and 50" versions of both of these Panasonic TVs, at Curry's. I've not seen them on standard or high-definition television broadcasts, they were showing either only the shop loop service or Blu-rays. I must say that I was very impressed with both of them. Without a side-by-side comparison it is impossible to be specific, but I think that, for Blu-ray viewing anyway, both of these are compable to my Pioneer plasmas. If you don't need Freesat then the ST30 series usefully cheaper than the same size GT30 series, and is plenty good enough.
Besides from the freesat, the ST is deeper, has a grey frame, rather than black, it doesn't have a THX mode and has a slightly more reflective screen (only an issue in bright rooms). Also and more imporantly is black levels aren't as good as the GT, but it's still an impressive TV.