Of course, the juicy information about sales breakdowns between models is, as usual, not reported. If I had to guess, I’d say the majority was 5s. I don’t think the 5c would attract an opening burst of sales in the same way the flagship phone does.
An indicator of the sales split could come from Apple’s newly-raised gross margin guidance. Assuming the 5s has better margins than the 5c, higher gross margin estimates implies a higher mix of 5s sales.