Both are good options, depending on the gameplay style you prefer. GO is obviously more modern. CSS has some redeeming values and a lot of fun minigame servers, classic maps, etc. I think CSS is a more challenging game than GO, and doesn't regularly offer a deathmatch style game, which CS:GO does. I suggest both, if there's a cool combo pack on steam
Source was my least favorite and I only bought it just recently because it was on sale. I played CS 1.6/DOD 1.3/Half Life 1 Deathmatch, completely skipping CS:S because I did not like it at all. I continued playing 1.6 for a long time, and only got back into CS with CS:GO. I did play Condition Zero for the campaign though years ago.
I didn't jump ship because source started dying, but rather because cs:go kept on improving until it surpassed source. At that time, eGO didn't have any cs:go servers anymore so I played a lot of competitive. After playing so much cs:go, I can't even play well on source anymore.