PC is a piggyback computer that modifies the fuel values of whatever map is programmed into the ECU.
Tune ECU/Tuneboy allow much greater flexability, in that much more can be changed( IE rev limiter, speed limiter, idle speeds at different temps, resetting of the TPS, MAP fuel settings) & is thus an essential tool.
SO why bother with PC? Well, there's 3 reasons:
1) lots of dyno operators only know how to work PC tuning.
2) PC5 can be fitted with an auto tune module, thus eliminating the need for a dyno, esp good for those with no access to a local dyno; + it's real OTR tuning.
3) Should you go for the Carpenter package or similar mods, the PC allows the injectors to open up to 100% flow ( stock max is 80%) & You'll need more fuel.
Can you use both? ie the tune ecu for the base program & a PC module as well? Yes! most of us who use PC modules do this.