USB serial adapter or serial PCIe card?

Which would be best/most reliable? A USB to serial RS232 adapter or a PCIe serial RS232 card? …and why?

Ok, to answer my own question, I actually extensively tested both now, a USB serial adapter and a PCIe serial card. The USB serial adapter which had a FTDI based chipset gave me driver problems on a few occasions, so it was not optimal.
The PCIe card worked well without any problems once the drivers were installed correctly, so I guess that makes the PCIe card the better choice.
Another option I also tested was using a serial Ethernet server which actually worked pretty well too, however it was harder to get configured correctly.