18f4550: Full Speed, USB 2.0
FTDI: Low, Full or Hi speed USB 2.0 depending on the chip. (
http://www.ftdichip.com/FTProducts.htm)
I haven't used the FTDI chips so i can't say much about them. Think about the PIC has native USB but the FTDI behaves like an UART, therefore you are limited to only a kind of possible configuration whereas the PIC can be configured for the 4 different working modes of the USB.
Resuming, FTDI is intended to simply the interfacing and focused in the communication and the PIC is real USB so you can create different kind of USB devices (HID, CDC, etc.).
If you see a PIC with USB support surely it's being used for REAL USB communication, not like a rs232-usb conversor (for that the FTDI and other chips were invented). Of course you can use the PIC in such a way but... it's a waste unless you need special features.