I say this as an R4512 owner...there is absolutely zero comparison between the two and if you're honestly considering both then I'd submit that you need to spend some more time identifying your needs and priorities in a saw before committing any money.
The Powermatic, while old, is a true cabinet saw, and the one in the picture looks well kept. I'm almost positive you'll find it has cabinet mounted trunions, which will make dialing it in fairly easy (assuming it's not already). Unless you're doing some pretty serious heavy work, like throwing 100's of bf of 8/4 oak or purpleheart at it, you will find it has plenty of power. Easiest way to see is to take some stock with you and ask the seller if you can try a few rips. At $600 its a steal if you ask me.
The R4512 is a good hybrid/hobbyist saw. My main reason for purchasing it was a lifestyle thing. I am active military, so I move every 18 months - 2 years. My "shop" is my garage, and when I'm not working it has to be put away because my wife has this illconceived notion about parking vehicles in there. So I wanted something that was light, with a good integrated lift. The lifetime service agreement from Ridgid was a big plus as well in putting the R4512 over the top once I had decided to go the hybrid saw route.
In a few years, when I'm done with the Army, or when the Army's done with me (whichever comes first) and we are able to buy a house I will be building a dedicated shop and at that point I will upgrade to a true cabinet saw (likely a SawStop).
Assuming you don't have the need to regularly relocate your saw, you can never go wrong with a cabinet saw, and there's a reason Powermatic has such a great name, they've been building great stuff for a long time.