ralphd: I think the third option you forgot is the cheapest - the PL2303. ... I have a whack of these after I made a deal with Roman at dipmicro to take 70 that he was having problems with for ~25c ea.
So - what were these "problems"?
They were clone PL2303HX chips, and would give "Error code 10" if you used them in windows with the latest drivers. The error was only because Prolific figured out how to detect the difference between one of their own chips and a clone chip, and updated their driver so it would fail with the clones. The Linux driver is part of the kernel, and I'm sure a patch submitted to make the driver fail on suspected clone chips would never make it into mainline, hence the clones work fine on Linux.
Once you go back to the old version of the driver, they fine on windows too. I did most of the testing of my half-duplex software UART using the PL2303HX modules on a Windows 7 box. They worked great even at high baud rates. The old Windows driver has a bug that will blue screen the machine if you choose a baud rate of 9999999kbps, but who needs to use that baud rate anyway?