Here is a brief explanation of each point.
USB to Host and to Device
'USB to Host' allows the instrument to be connected to a computer via USB, typically to send and receive MIDI data via a single USB cable.
'USB to Device' allows a USB device (e.g. a USB flash memory stick) to be connected to the instrument, and for data to be loaded into or saved from memory.
256polyphony instead of 192
Polyphony refers to the number of samples that can be played simultaneously. In the past, lower polyphony specifications (such as 32 note) could hamper playing, as sustained notes would be 'dropped' when new notes were triggered. However, this should be a thing of the past with 192 and 256 note polyphony. Generally speaking, the higher the polyphony number, the more powerful the tone generator of the instrument. 256 notes is a high specification, and should that notes are never dropped - even when playing lots of chords with the sustain pedal, using layered/split sounds, and a full rhythm section accompaniment all at the same time.
This ties into the 'USB to Device' functionality mentioned above, whereby pianists can make a high quality audio recording of their performance directly to a USB memory stick in WAV or MP3 format. The saved file can then easily be copied onto a computer and posted online or emailed to friends and family.
The Amp Sim (amplifier simulator) functionality attempts to recreate the acoustic character of vintage amplifiers, which were commonly used with electric pianos in the 60s/70s (and still today!). Applying an amp simulator to an electric piano or organ voices (i.e. not acoustic pianos) can help to improve the authenticity of a particular sound.
General MIDI 2 refers to a standardised group of instrument sounds (and effects). Many .MID files downloaded from the internet are created for playback using a General MIDI or General MIDI 2 compatible device, with the goal being that a MIDI song created on one device, will sound more or less the same when played back through another device.
If basically the keys and the sounds quality is the same, is it worth to spend $650 more on CN 34?
If you believe the additional features and functionality will be useful over the lifetime of the instrument, yes, the additional cost can be easily justified.
How different or same is Yamaha 440 to that of CN 34?
There are some aspects that are very similar, and others that are very different. The best thing to do is play-test both models for a few hours in order to understand the characteristics of each instrument, and which is more suitable for your needs.
I hope this helps.