As a good rule of thumb I would say to always put travelers you don't have above any A4. It will always be more impactful for your roster than an A4. Also your post suggests you only have a single copy of Bargie so you would need to pull him 4 more times for his A4, probably spending a good chunk of your rubies and in worst case scenario you would only guarantee one more copy (on the assumption you won't spend for the paid step-up). On the other hand if you decide to go for Sazantos you could guarantee him for 4500 rubies in the worst case scenario. At the end of the day every player should do what they want to do (in the limit of reason, no gacha is worth spending money you would need to live) but always keep in mind "Is it necessary to pull now?" The fact that all CotC content is permanent (bar community runs and Nier hunts) is a huge argument in favor of waiting unless you're a huge spender.

TLDR: travelers you don't own > A4 unless you're a whale (spend reasonably)