I am using both Capitalspreads and Worldspreads. Both have their advantages and disadvantages.
Capitalspreads have no automatic execution. This requires that the trade first be approved by a dealer. If you are a short term trader like me this simply does not work. I want to get in and out very quickly and like to have instant verification of my trade. I have also found that the overall speed of the Capitalspreads platform is way too slow for my requirements. However, the platform is very user friendly and contains many features in order handling that Worldspreads at the moment are missing. One such handy feature is being able to close the position just by clicking close and confirm. What bugs me about Capitalspreads, however is that if you have the dealers ticket window open, you get timed out after a few minutes, meaning that the window shuts down and you have to open it again. The support department at Capitalspreads is also excellent with very quick responses on your questions.
Worldspreads have instant execution, this feature alone together with 1 spread (Capitalspreads have 2-3 spread) on the instruments that I am trading makes them the right choice for me. The platform is extremely fast, one gets the feeling of trading with direct market access (DMA). Not having charts might put some newbies off but again I like to have my own charting package provided with feed from the direct market access. The web interface is also very clean with a big display on the quotes in the dealers ticket window, this ticket window does not time you out. One extremely important feature, which, for the most part is overlooked, is that the feed should as closely as possible follow the underlying asset. This way, the trade is much easier to monitor, especially if you have an external chart package. Worldspreads follows almost exactly the underlying asset on the 1 point spread instruments, this cannot be said for Capitalspreads. The support and order handling, however needs to be improved with a stop loss ability in the order windows and also OCO (one order canceling the other).
Good luck with your trading!