Why would one be better than the other?
The serial data lines have +V as their idle state, so you want to wire the LEDs so that there are "off" when there is +V (active), and ON when the signal is at 0V (active.) You can do this by connecting the plus side of the LED to the +V power supply, and the cathode to the signal, rather than the more common (in arduino-land, anyway) method of connecting the LED cathode to GND and the anode to the signal.
Note that if you have a "real rs232" signal, there is an inversion that is part of the spec, so rs232 breakouts go back to connecting between GND and signal (the signals then also go BELOW 0V, so two LEDs are good.)
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