You'll probably find that:
It will not regulate the voltage, therefore you will be putting 14ish Volts into your HDDs etc
It will restart your computer when you start the engine
It will not automatically start up/shut down the computer with the ignition
Now you've got that, you'll probably have to spend another $200 for an Opus when you find it's not good enough. Instead of only having to save another $130 for the Opus

Also, that PW120A is just a PW70A with one changed component...and I personally wouldn't trust it as the rest of the circuit is only designed for about 70W
Garry