One thing to be aware of is that USB-serial adapters require conversions on both ends, serial-to-USB & USB-to-serial. This can be a factor in time critical applications. IOW, 9600bps will not be a true measure of throughput. I have encountered problems with some bootloaders when using USB-serial and, also, with ethernet-serial adapters. Also, while not a factor here, some USB adapters do not handle all of the modem control lines like DTR, etc.