From someone who started on 1.6 I'd say source has my best memories, but the community for css died with cs:go. Play ability I would give to source, I preferred the simplicity of it.
If you're looking at joining a team and taking things seriously then I would suggest going for cs:go as most css teams are going to be of a high standard whereas cs:go will have many lowskilled ladders for you to build up your skills.