You new a new blu-ray player and a new receiver.
If your receiver is older, it will not have TrueHD or DTS-HD decoders that are on most blu-ray discs.
Your old DTS decoders will decode the DTS-HD sound into regular DTS sound, but it will not be able to decode the TrueHD sound. If the blu-ray disc also has Dolby Digital Plus or Dolby Digital, then your old DD decoders will work fine.
If your receiver has the 5.1 CH analog inputs, then you can let your blu-ray player do all the decoding and send the final signal to your receiver.
Which one is better if you had the option? Probably doesn't matter. But I prefer to let my receiver do all the processing.