I've owned the FN Five-seveN mk2, the Ruger 57, and the PSA Rock 5.7.
The FN is a top quality pistol, no question about it, no regrets on owning (or carrying) it. The real surprise for me was how much better the PSA Rock 57 is than the Ruger, and in some ways, even the FN.
Once I got my hands on the PSA the ruger went out the door maybe 2 or 3 days later. The PSA does everything the ruger does but better, and for cheaper. I've got several of the PSA Rocks now and I have not had a bad experience with anything about them, which is not something I can say about the Ruger-57's I've had.
I took all 3 to the range and the trigger on the PSA was vastly superior to the ruger, and was honestly better than my FN.
Accuracy was great from all 3 guns, but using common (and cheap) glock sights makes the PSA a winner here for me.
Magazine capacity is an obvious win for the PSA, although I've had decent luck with the pro-mag 30 rounders in my fn, ymmv (all the ruger mag extensions I have seen are a joke, and a terrible joke at that. F5 mfg is forever trash in my mind now.)
Price for the PSA starts at 500, Ruger is around 600, FN is OUCH, another win for PSA.
Magazine design is one area where I 100% prefer my FN. I run some specialty loadings for my five seven stuff, and the FN magazine design is just better at preventing hiccups (specifically the front of the rounds crossing eachother), however I have not had any issues with factory ammo in the Rock...yet.
The grip and design on the PSA is also great, and most folks I've handed it to have been really impressed.
TLDR; if you're going to buy the ruger 57, buy the PSA Rock 57 instead.