USB to Serial cable and VCC current output
The difference is that some cables have a built in LDO (linear regulator) supplied from the USB power for 3.3V that can support up to 250mA, whereas the others use the built in LDO on the FT232R. Quoting the FT232R datasheet:

+3.3V output from integrated LDO regulator. This pin should be decoupled to 
ground using a 100nF capacitor. The main use of this pin is to provide the internal 
+3.3V supply to the USB transceiver cell and the internal 1.5kΩ pull up resistor on 
USBDP. Up to 50mA can be drawn from this pin to power external logic if 
required. This pin can also be used to supply the VCCIO pin.

In general, if you are using it to flash the firmware of some set-top box, it's unlikely you will need power from the USB anyway, since all the components on the set-top box should be powered by the power supply of the box (be sure, of course, to connect the ground through the RS-232 interface). In that case, I suspect your answer is just whichever cable is cheaper.