The PW70 doesn't regulate the voltage. The ITPS does, however it has a 1.3V dropout. Which means you need to pump at least 13.3V into the ITPS to get 12V out the other end. When your car is off...it won't give out that power.
The Opus takes voltages from 8-18V, has the startup/shutdown controller, regulation, more power (the ITPS will only output 60W if it's regulating), and it's one individual unit, designed for the job, rather than two seperates hacked together to do a similar job.
There is a group buy happening on the Opus for $182 plus shipping for the 150W model. It is normally $190 plus shipping if you wish to order direct from Opus. I am unsure on the prices of the 90W model.
Yes, it is more expensive, but it's totally worth it IMO. Next to the screen (which should be VGA) it is the most important piece of hardware in a carputer. With no power, you ain't getting nothing!