Don't know much about the Westinghouse units, but on the plasmas, yes there is a difference in the pic on the 60 and the 600. For my money, the Pioneers look the best hands down on the plasmas I have looked at with SD content. Are they great? No, but SD will not look stellar on large screen tvs in general. The Pioneers looked the best to me, however. You can get a 42" in your price range, but not a 50".
As far as burn in, SD or HD makes no difference, just having static images on the screen for long periods of time. One such example is watching ESPN or news channels a lot with the bar at the bottom of the screen. I have seen tvs in stores that use ESPN too much that have the logo and the bar burned into the screen.
You should be fine, though, as long as you watch diff channels and keep the brightness, contrast and sharpness down a good bit, especially for the first 1-500 hours, depending on who you ask.
The OPPO 981 is a super machine. I have it matched up with my Pioneer 50" plasma and the pic is exceptionally good.
Good luck with your purchase and welcome to the forum.