I will base my answers on stats, moves, Kahunas, and when it is obtained.
Salazzle is better in terms of typing, as it is not weak to dark. It is definitely less bulky, but has more versatility and much better attack potential. They are both obtained at around the same time in the game. It might take a few tries to get a female Salandit, but trust me, it's worth it. You get good early-game moves, but it lacks coverage. You also get a very good ability. Salazzle is best kept away from the Kahunas(except for the Dark one. You can't Salandit it before the fighting one I think), but it's a bit better in my opinion. Also, focus sash toxic can make the Ultra Necrozma fight really easy.
Alolan Marowak is a lot weaker in terms of attack, so you will want to overlevel for any Kahunas or major battles. However, anything that's not a physical attack is going to leave a massive dent in your health bar, ESPECIALLY if it's a super effective move. Alolan Marowak is weak to all the Kahunas that you can face using it, and every single one of the Kahuna's Pokemon have moves that threaten a OHKO(unless it's physical, in which you survive with like 5 hp). It has lots of ground moves, but you probably won't even survive to use them at any degree of effectiveness. Also don't even try to use swords dance, you'll just die before you get to get any good hits.
Salazzle is generally better. Hope I helped!
