Comment which is better! :D
1.6
CS:GO
I like 1.6 better, but then again I played counter strike when it came out 1999 so I'm probably biased.
What you should consider is the people who play 1.6 are mostly hardcore players, also the original cs has very barren maps which means it's very focused on reaction time, precision and an analytical knowledge of the maps.
I found that counterstrike source had lost some of the "counterstrikeness" : the reason for this are the nicer graphics which make it harder to spot opponents at a distance which changed to game-play, and made sniper-rifles more powerful.
Can't say anything about cs:go haven't tried it.
1.6 by far.
It's all personal preference. 1.6 is for the serious more hardcore parkour, where css (from what I see) has the servers which you don't take so seriously. Also, css is good because it has a lot of files that Garry's Mod needs for it to run at it's full potential (TTT, Prop Hunt, etc.).