Luke vs Vader was Vader trying to seduce Luke to the Darkside--with Luke taking in a somewhat exhilirated and flat tone, hardly emotional. Then it switched to Luke getting all pissed and hacking at Vader, also not emotional.
Obi and Vader had very little talking until you get a hopeless Obi saying he lost and failed his "brother" and said "brother saying" he hates him. Plenty of emotional voices and faces.
Obi and Vader are much more heart-rending.