Older HT receiver used vs. new HT receiver

I'm having a difficult time deciding whether to buy a HT receiver 2-3 years old which offers THX, DTS and Dolby Digital or a newer HT receiver that may have some of the newer surround formats like DTS-EX, Neo6, etc. I see receivers like the Marantz sr-18/19 (not EX), Denon 4800, or Yamaha DSP-A1 for sale in my price range and newer receivers like Denon 3803, NAD T752, or Marantz 7300 also in my price range. I currently have M&K 55's L,C,R,surrounds and a subwoofer with no immediate plans to add a sixth speaker in back. This may be a stupid question, but any help would be appreciated. Any recommendations would be helpful. Thanks