Ultimately, it’s up to you! Depending on what your character needs, either an ASI or a feat may be more useful. I’m currently running a level 3 Barbarian and am considering taking either the Tough feat, for +2 extra HP per level, or increasing my constitution by 2 to gain +1 HP per level, but also increase my constitution saves by 1 as well as my AC by 1.
There are no wrong answers as long as you’re building the character you want to, and sometimes taking a feat to add flavour, new options or versatility is better than taking the increase to stats.
Well that’s everything about ASIs and feats. Hopefully, you’ll now be able to make a more informed decision the next time you level up. Keep an eye on your class’s feature table to see when an ASI is coming your way, and get planning your progress!