I had the same question when I bought mine. I ended up going with the FNS. Don't regret in one bit. Very accurate and reliable. Only reason to get the X imo is to cock the hammer ahead of time for a feather trigger. Since that's not important to me I went with the S.
The triggers on these are very nice too. I think its a win, win situation honestly, but I love my S. And $659 doesn't seem bad to me. I paid $629 on sale for mine and that was before all the rush on guns happened. I've seen them as high as $699 now for the FNS-9mm, so at $659 I'd say that's about right.