there are good and bad points to each of the three main choices..

1)cygnal f320
It only supports interrupt driven transfers.The drivers are not the best ,neither are the all important examples ,and the f3xx series does not support conditional breakpoints (imposed by the use of the C2 interface) but does have the C2 interface to debug with

2)cypress EZ USB
supports interrupt and bulk transfers , drivers and examples are excellent..ports are not standard,uses PIO

3)external FTDI by DLP design etc
good drivers for PC side (with VB ,C, and Delphi examples) easy to use, can be bulk written to ,but takes 1ms per character if used in this manner..and is an external board/chip..