Hi
I would recommend you to get the Sony HT-M5, sound wise there is not much difference between the two models. But the Sony HT-M5 is a newer model and it has 3 HDMI inputs when comapred to 2 HDMI inputs in Sony HT-DDW5500.
Both the systems do not support Dolby TruHD and DTS-HD MA. But you can get Dolby TruHD and DTS HDMA sound quality in Sony HT-M5 if your Blu-ray player can decode Dolby TruHD and DTS-HD MA. The Samsung player that you have can decode Dolby TruHD but not DTS HDMA. So when you connect your Samsung Blu-ray player to the Sony HT-M5 you can Dolby TruHD quality sound. If you want DTS-HD MA sound quality through a Sony HT-M5 then you should get a Blu-ray player which supports DTS-HD MA audio decoding. The Sony HT-DDW5500 does not support both the formats even if your Blu-ray can decode them.
As of now most of the movies that you download from the web will only have normal Dolby 5.1 or DTS 5.1 sound. If you want to get DTS-HD MA and Dolby TrueHD sound then you will need to play Blu-rays which have these formats.
Also Dolby TruHD and DTS-HD MA are 7.1 format, which means that they have two extra surround speakers at the back and it will give better surround sound effects. In future almost all Blu-rays will have 7.1 sound and also Aitel DTH will have channels with 7.1 sound. But these Sony systems do not support 7.1 sound. But you can also enjoy Dolby TruHD and DTS-HD MA sound quality with 5.1 system but the surround effects will be better in 7.1 system.
If you want 7.1 sound then look at some other brand. Onkyo has good 7.1 home theatre system - Model no : HT-S5400 and it costs some where around Rs 55,000 - 60,000. But it won't be as powerful or loud as the Sony systems but it has better features and you will notice a bit more clarity in sound. Should be enough for home use but not for Sangeet during the marriages, Office parties, Hotel parties
If your not bothered about not having 7.1 sound then get the Sony HT-M5.
Your home theatre system will tell you in which format the sound is coming. Like if your getting DTS sound then the display in the home theatre system will show DTS.