Bourdais is a very good driver, maybe too good for CCWS (I am not bashing CCWS here). But with all respect to his incredible records, some of the top-drivers in the USA are in the rival IRL series, so I don't know whether Bourdais would have been that successful in a unified Formula, or would have been great, but not that dominant.
But really it is better to be record=-breaker in CCWS than 20th in F1 (which he will be if he is in STR), so I think that he'll stay in the USA.