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The 5200 has a more superior sound output than the 3100. So whether it is worth it is really up to you, and depends on the size of your room, how good you like your sound, etc. In my opinion it is definitely worth it if you are going to be watching blu-ray movies on a regular basis and you want the best possible sound.
The best 48" Sony HD Smart TV is the KDL48W600B, and will set you back around $1000.