If you connect your BlueRay directly to the TV and then output the TV to the soundbar, you will get two channel audio only as most TV's do not pass multi-channel from external sources (thank the music industry and digital rights management for this little bit of stupidity).
If you connect both the TV and Blu-Ray directly to the soundbar, you will pass the audio directly from the source, whether 2-ch, dolby digital, dts, etc.
I have listened to both the Panasonic HTB-350 and HTB-20 and ultimiately decided on the HTB-20 as it seemed to do everything as well as the 350 at a cost to me of about $ 100.00 less. After leaving the HTB-20 in place for about two weeks, it is now our primary device for audio from video sources. It does lack the blue-tooth connectivity available with t 350. Not sure if the 450 is the same as the 550. Have not seen in person or heard either of these units.
One caveat on most Panasonic soundbars is the subwoofer (or actually a woofer as I don't know of any soundbar less than $ 1,500 that comes with anything approaching a subwoofer). The two models I auditioned came with the woofer level set extremely high making the bass sound muddy and overwhelming. I turned the bass down almost to zero and increased it until it seemed in balance with the soundbar. I have not attempted to calibrate using test tones as I have not had time. I also turned off the 3-d effects as they seemed to add some sonic anomalies in my room. The simulated surround seems to work quite well as it throws an image far to the right and left of the actual device.