I use both. I main Ness because he's just better in more situations. There are some differences you should be aware of to help you choose which is better for you in certain situations.
Ness is straightforward in his combos and kills, and deals lots of damage very quickly. His juggle game in particular is completely broken. However, he is vulnerable off stage and isn't very fast.
Lucas has a tiny bit more range, better speed, an amazing, varied recovery, and absolutely insane edgeguarding. His PK thunder can absolutely decimate the likes of Lucina and Shulk off stage, both characters that Ness can struggle with. However, Lucas has very limited combo options and can struggle to kill.
Basically, use Ness when you're confident you can get in and overpower your opponent. You can get early kills without too much effort and build damage quickly.
Use Lucas if you want to play a more evasive, defensive game. You will mostly rely on chipping away at foes with PK fire, zair, and fair to deal damage, but this can be made up for with good edgeguarding.