I am going to differ a bit from my fellow forum members and suggest you sell neither pistol. At least right now. The current pistol market is very much a buyer's market, and the FN 509 is not popular enough to provide much in the way of funds. Gunprime is selling new ones for $465 shipped to a FFL per
https://gunprime.com/product/fn-509-...oly-66-100002/ and $430 per
https://gunprime.com/product/fn-509-...k-66-100005-b/ . So a used one is likely to bring less than $300. Rather than take a loss right now, I suggest putting it in your safe and starting your formal training.
In my mind, most recent pistol designs are good enough for most uses. The things that differentiate the offerings for me are manufacturer support and aftermarket support. For those reasons, Glock makes a lot of sense as a platform and FN does not. But choosing one platform for carry need not mean abandoning the other. There is value in a pistol you feel you shoot well even if it lacks in popularity. If you can afford to keep it, I would do so. In the end, pistols are cheap. Shooting them well is expensive.