One option, and maybe your best, would be to find a good retail store that carries both TVs. See if they can connect SD to both. This might be tough because 1.5 years ago when I bought a Sony LCD for my bedroom, I tried to do this hookup scenario at both Circuit City and Best Buy, and neither had an SD signal source hooked up to compare. No fault of theirs as they just didn't have an SD antenna available. But you might luck out and find a retail store that does have both TVs and an SD source.
Another option might be someone here who owns one of each TV, then maybe they can reply on how their TV looks on SD. For me, I have a smaller Sony LCD in he bedroom, and it's OK on SD, but not as good as I would like, but watchable. I just bought a 55" Sony SXRD 1080p for the TV room, and it does a great job on SD. Of course that doesn't apply to the Sony XBR4 because that set is a flat-panel LCD and my Sony is a LCD RPTV, and even if both TVs use the same video scaler component, a Flat-panel LCD is a totally different display. My friend just bought a 58" 1080p Panasonic Plasma, but he won't show me SD on his set, only high-def channels on DirecTV or his Blu-Ray. Not sure why.
I am undecided on which of these 2 tv's I want to purchase. They both look great but I am unsure of their standard-definition performance. How do they measure to traditional tv in this respect? Which one would you recommend based solely on standard definition performance?