Regarding FX, these days you can trade at interbank prices with non-dealing desks like MBtrading. Their spreads on EURUSD, USDJPY, GBPUSD, EURJPY, EURGBP and USDCHF are moslty 1, but you can trade within the spreads and the fills are superb. You can get 1-2 pips on USDCAD as well. The commission on pairs with USD the lead currency is $10 per round trip per lot ($10/pip) and about $13 on EUR/XXX and about $18 on GBP/XXX . You, of course, pay tax on the profits.
So, it depends on the pairs you trade. Simon offers 2 pip on USDJPY and EURUSD, for example. On the face of it, therefore, you won't do better trading interbank that with CS on the two majors. For GBPUSD, you are paying about 2.8 pip in spreads trading with MBtrading, whereas Simon offers 3. However, you can sometimes get excellent spreads on pairs like GBPJPY, AUDJPY and USDCAD trading direct. You also get 1 pip on USDCHF with MB trading most of the time.
Now, if cost was the only consideration, I will trade the majors with Simon's outfit and forget about the rest because the tax free incentive plus the tight spreads will make it the thing to do. But (a big BUT), the excution and the feed on CS can be really terrible. If you want to go in and out with 10 pips, then it is much better to trade direct. If you are looking for 30 pips or more, it clearly makes no sense to pay the same amount in costs and then pay tax on the profits.
Having said that, I find it psychologically easier to trade direct because feeds with CS are a bit wanting. If they sort that out, the direct access guys will have to reduce their commission drastically to compete.