Must say that I find that absolutely astonishing. I know that different brokers have rather different commissions/spreads on the same instruments, but that seems a really _enormous_ difference to me!
I'm a simple forex and indices trader (mostly forex, these days). I don't have an account with InteractiveBrokers. I have one with FXCM and one with CS, and I can only tell you that on the currencies I trade, the CS spread is always smaller than the FXCM one.
To me, comparing small differences in the spreads/commissions pales into insignificance compared with the difference between paying CGT on the profits and having them tax-free. But people's situations and needs differ, obviously. To be honest, I'm pretty surprised, given what you say, that you should be considering using CS at all.
Happy New Year and a successful 2005 trading.