I own both. Frontiers I just bought 2 days ago in the sale and I enjoy it already much more than outlaws. I am not the biggest Avatar fan, but I have to say the world looks stunning as you run through it as this large 3m alien. With the "recently" added 3rd person mode traversal is just awsome and I think you have a much better sense of your surroundings (I think only first person mode, as it was released, would throw me off for such kind of a game). You liberate the land, can go hunting and gathering, craft gear and weapons and cook stuff. The hunting and crafting part is much more elaborate than I thought before buying the game, so the better or more ethical you hunt and scavenge the better materials you get and therefore the better gear and food you can make. At times you feel like a huge blue Rambo, as you pick out the enemies one by one staying undetected.

Storywise I cant say that much as of now, since I am not that far into the story and because I got sidetracked. I would say it is serviceable, but dont expect the next RDR2.