CMC have always had better FX spreads than CS as far as I can remember (although I know CS tightened their spreads substantially recently)*. Use the right tool for the job. 0.4 point ES - CS. 1 point FTSE - CS. Tight FX, and 24h prices on all FX pairs - CMC. CS doesn't quote certain FX pairs between 9pm and 7am, which can be a bit of a pest.
As far as the spread widening - I would assume that there is less volume on this instrument during the night, and as clients are not trading as much CS are more likely to need to hedge as they don't have a large number of clients with diverse views offsetting each other, so they need to protect themselves with a wider spread. CMC have more clients than CS (I think) or can bear more risk (I think) and therefore don't need to widen their quotes. I also think CMC is active in the FX market, and can most likely hedge cheaper than CS.
Just my best guess until Simon replies. Also, does anyone else notice that CS quote the wheat per full point rather than per 1/4 like the other SB firms and the underlying?
* - this isn't too much of an issue any more, but I remember when GBPJPY was 10 with CS and 8 with CMC, and the same for some other crosses.
