I'm designing Desktop Chat app, but I see most of it using list arrangement eg. Facebook & Telegram than left-right arrangement eg. Skype.
which one the best for readability?
List arrangement is good for readability as the eye fixation does not need to wander between left and right often. However, lets see the context we are in. Its a chat arrangement, in a desktop.
1) Finding information can be tedious in a List arrangement: In a well populated chat conversation, finding for a piece of information/reference can be difficult, as you will have to scan through what both parties said. In reality we do try skipping our own parts, but its not easy. However, if you have a left-right arrangement, then skipping one side is very easy.
2) If there are no avatars (profile pics) - then list arrangement becomes difficult to navigate through, unless the design really takes care of such things.
3) On a psychological level - A conversation/chat is supposed to be promote a more casual, instantaneous and human feel to the online discussion. Its a technical replica of two or more people sitting across and talking. Left/right arrangement kind of provides that, by creating separate space for both parties, as if sitting just across!
I'll vote for left-right arrangements - but will also mention that there is ample scope of design improvements there, compared to common references you shared.