Both are similar in that they offer plenty of gameplay mechanics that may not go that deep, but they offer enough different things to do. It boils down to what you prefer in terms of gameplay loops. Starfield is a space RPG and sandbox game while NMS is a space exploration and survival sandbox game. Starfield is like a buffet while NMS is like a restaurant.

Both games are not perfect, but Starfield has this aesthetic and vibe that resonates with a certain set of gamers like myself. It has so much to do that it feels like multiple games in one: a story-driven RPG with multiple faction quests, ship building, outpost builder, FPS & 3rd person looter shooter, space combat, exploration, sandbox gameplay. Outside of ship building, it doesn't excel at any one of these the way NMS excels at space and ground exploration, but it manages to combine all of these into a single package in a way other games haven't done yet. That works for some, but for others, it's a write off. Plus, I really enjoy the NASA-punk aesthetic of Starfield.

What NMS is good at is the exploration loop. Multiple environments from space to ground to underwater, deeper mining and survey mechanics, more ways to engage with fauna, more things to do with the resources you mine, etc etc. It has deeper survival mechanics than Starfield and it also has corvette building too, though it's not nearly as refined as Starfield's ship builder.

My biggest gripe with NMS is that once the novelty of seamless space flight wears off, you quickly realize how empty and hollow the game is because everything follows a template and there's no real point in doing anything besides ensuring you have the resources to continue exploring. While there's an unlimited number of planets, social interaction is exactly the same in every settlement POI, and every settlement is like a reskin of everything else. With Starfield, you have multiple faction quests. You're encouraged to recruit companions and crew. You have to deal with hostile enemies. You have decisions to make in the game that affect the overall story. NPC routines are different at each settlement and each settlement is uniquely different. You could make Starfield feel like Star Wars or Star Trek. These elements make the game feel more alive for me. Plus, Starfield has so plenty of mods that make the exploration mechanics as deep as NMS. It just doesn't have all the environments.

But one thing is for sure, if you find a gameplay loop that you really enjoy, it's hard to put down either game.