I have had both. I, personally, don't like slide-mounted safeties. I, personally, don't like DA/SA.
The FNX-9 is great because it can be carried like a Beretta or a Sig (decocked and safety off) or it can be carried Condition I (like a 1911, cocked and safety on).
Condition I lets you enjoy the same light, short SA trigger pull for every shot but requires you to deactivate the safety before pulling the trigger.
Carrying it like a Beretta or a Sig lets you just pull the trigger when you need to fire it. You don't have to worry about deactivating the safety. But, unfortunately, your first shot is going to be more of a challenge to pull off well because it is a long heavy DA trigger pull followed by the easier to use light SA trigger pulls until the gun is empty.
I would rather get a Sig than a Beretta because Sigs lack the slide mounted safety.
But, I would rather have an FNX over either because 1) You can carry it the same way if that tickles your fancy. 2) Condition I (IMHO) is better for better shooting.
My only complaint with the Beretta besides having to carry it in Condition III is that the safety is mounted on the slide. That makes it possible to accidentally switch the safety on when clearing a malfunction.
My only complaint with the FNX is that the base or most rearward portion of the safety dug into my thumb a little bit while firing because of how it sticks out. Its not a big deal because its plastic and I have a big thumb knuckle anyway.
Rifle: Custom AR-15, Tavor, Arsenal SGL 31-68
Pistols: Colt Python, XD(M) 4.5" .40, Glock 19, M&P 9mm, Walther PPQ 9mm, HK P30S 9mm, Walther PPS 9mm, Browning Buck Mark