The S1 is on less of a sale... The two series of 1080p plasma HDTVs are very similar similar aside from a few key differences. The S1 series has a higher native contrast ratio spec (40,000:1 vs. 30,000:1) and an antireflective coating on the screen, so we expect it to exhibit slightly better black levels and bright-room performance than the U1. The S1 also has slightly higher motion resolution (1080 vs. 900 lines) and a NeoPDP panel that uses less power, but these differences should have a negligible impact on picture quality.
Youd probly be just fine with the U1 series. I think that those mediocre changes dont hold enough water to spend the extra cash