I've used both, and it's hard to say whether one works better than the other. Anything connected to a Windows PC may exhibit the occasional odd bit of behaviour, but they both seem to work. The Silabs chip uses up much less PCB area than the FTDI chip, but you have to use an assembly house that can solder QFN packaages. The strength of the FTDI solution is in the flexibility it offers both on the endpoint interface and on the PC driver.