I've used both a PC and an ETS 150/3 for car polishing. Haven't tried my RO 150-FEQ yet because the ETS 150 does a great job, so I haven't felt the need.
I agree with Dan Clark that the Rotex mode on the RO 150 would be preferable if you had heavy-duty work to do (removing the clear coat or oxidized paint, for example).
I'm far from an expert, so take this with an appropriate grain of salt, but for general-purpose polishing I'd be surprised if there's any noticeable difference between the ETS 150 and the RO 150 in random-orbit mode. I'd expect technique and the choice of abrasive to be much more important.
Allen