AM2R, Samus Returns; which one is better? Well I'm not going to answer that question, but I am going to give my thoughts about both. Having not played either remake, I thought I'd play them back to back.
Brief background: Metroid 2 was originally released on the Gameboy, it featured Samus on a mission to hunt down and exterminate the Metroids, genetically engineered super creatures created by the Chozo race, used by Space Pirates to threaten the galaxy in the first game. The metroids transform several times in their lifecycle to become stronger, thus Samus will encounter progressively stronger variants of Metroid.
I played Samus returns first (11 hours 8 minutes, 62.7% items found), and then AM2R (5 hours, 6 minutes, 72% items found). Both good games, but with different ideas about how to do a remake. AM2R's goal is a Metroid 2 in the style of Super Metroid; retaining the original character of the game, but giving it a visual and control upgrade, making the Metroids more threatening, and adding some new content. Samus Returns is less concerned with new content and maintaining the character of the original game, and more interested in revamping the game. Metroid behavior is significantly changed, regular enemies have more health, do more damage, and are more aggressive, Samus has new abilities, and there's more focus on puzzles in the room layout and item locations. While AM2R gives the original Metroid 2 a modern coat of paint; Samus Returns only maintains the outer appearance of the original while being completely different under the hood.
One example of these two approaches can be seen in the Spider Ball ability. AM2R retains the original characteristics of Metroid 2's Spider Ball: You can't jump in Spiderball, and bomb explosions force you out of Spider Ball. Samus Returns revamps the spider ball and lets you jump, and stay in spider ball when propelled by a bomb. It's not as restrictive, and moving around in spider ball is smoother in Samus Returns.
Exploration is a major point of difference, and is largely why Samus Returns took over twice as long for me to complete. AM2R is more straight forward in its exploration, paths between major areas are simple hallways, items are usually just in hidden spots, or behind a single block you need the right weapon or ability to break. Samus Returns asks you to make more use of your abilities to navigate and find items; but only because it gives you a powerful cheat called the Scan Pulse. Scan Pulse fills in your surrounding map, but it also highlights all breakable blocks, and if a Metroid is nearby it will roar. It's a completely optional ability which you can choose not to use, but it does a lot of the hard exploration work for the player. Filling out the map to show room layouts is one thing, but highlighting blocks is, in many cases, equivalent to solving the puzzle for the player. On paper I understand that Scan Pulse lets players adjust the difficulty of exploration to their abilities; but that doesn't stop it from annoying me.
Metroids are another big contrast between the two.
Well this post has gone on long enough. As said, both are good games, and its interesting to see how they approached the same game with different goals in mind.