The BD55 is identical to the BD35 in almost all respects, including using the same high quality picture pricessor, Panasonic's PHL Reference Chrome Processor Plus and the ability to bitsream and decode all the high definition audio formats, including DTS-HD Master Audio. It is also Blu-Ray Profile 2.0, DivX and X.V. Colour compatible.
The BD55 costs more due to the following - New capacitors, a new 192kHz/24Bit DAC, tweaks to the HDMI output to help reduce jitter, and better insulation to keep vibrations to a minimum.
And yes, the BD55 also has a set of eight-channel analogue outputs, for owners of older amps.