If you go with that itinerary or something like it, having flown both CO and CX in all cabin classes in the past, I would give CX the nod over CO unless you have some sort of fear/extreme dislike of JFK. (which is not irrational, by the way) I respect CO BusinessFirst, it's a solid product at a solid price but at least imo CX C is miles better than anything any US carrier can offer. I'd even rather fly CX in C over UA in F. Service, IFE and lounges will all be far better on CX over CO, I'd rate food about the same (one of CO's strong suits). With that being said, CO is *A and they'll fit into a *A RTW nicely.

Btw, isn't a westbound RTW "backwards"? I'd do it eastbound.