What is SD PQ like on 43" vs 50" pioneer plasma's ???

[chaparral]

Am looking at upgrading my pioneer 436xde 43" plasma to one the new pioneer 508XD (720p) or LX508D (1080p) plasma's..But am a little worryed about what SD looks like on the larger 50" screens.

I have skyHD but about 75% of my TV viewing is still SD
My viewing distance ranges from 9ft to 11ft....

Would there be a big lost in PQ with SD on a 50" vs 43" at my viewing distance ???