Two different animals
The ProLine arms are great they give you the extra width and are rigid, therefore you get great control. I have seen broken ones before but have beat the living hell out of a set with no breakage.
The RPM are extended as well but made of a much softer plastic compound. I have never seen one brake but you loose a bit of control due to flexing.
There is no such thing as a free lunch, to get you have to give. Just make the proper choice for your driving style.
If your just out bashing and having a great time either set will treat you great.
I'm sure others will have a different opinion. I own both sets and love them.