The FNS is the Glock Killer, donchaknow? All of their serial numbers start with GK because of that.
I'm not saying I agree with that... it's just their own little jab.
One is striker fired, one is DA/SA. 2 different trigger types, basically the same ergonomics otherwise. Pick which one you like better. For a striker fired gun, the FNS has a fairly high bore axis. If I was going to accept the higher bore axis, I'd probably take the Sig P320 instead.
The FN's are solid guns, though I hear their customer service is lacking. No personal experience with that.