All of the KW's that you listed are fantastic. But they are all different styles:
They are all open case back showing the same movement. I have the 388 and its truly a fantastic Rad, boarders super rep status. Because the movement replication is not and probably will not ever be 1:1 it will likely stay a high end 1st class rep. (Same goes for all three that you listed)
That being said - the 505 is the same as the 388 just PVD and has a nice brown dial.
The 312 is a completely different shape but same level of craftsmanship.
In regards to your choice - its really all about your taste at this point. All three models are fantastic and you will like each of them very much. If this is your first pam, i would suggest going with the 312 as it will have the infamous Crown guard that symbolizes Panerai.
Hope this helps a bit