Underfell:

-Flowey is nice towards the player. From what I seen in the Underfell arts, the player can carry him around.
-Monsters wear clothes with red, black, and sometimes yellow color schemes.
-All of the monsters are mean towards the player and may or may not want to kill the player.
-No designated plot; the AU is open to interpretation towards people who are interested in it.

Negativetale:

-Flowey is nice towards the player. Unfortunately, the player cannot carry him around. He can only appear in certain locations.
-Monsters retain their personality but are driven by negative emotions like hate, fear, guilt, or disgust. We only know two monsters in this AU so far. Napstablook is shy but hates to reach out towards others. Toriel is an overprotective mother who murders anyone who may try and get close to the player to keep the player safe.
-Plot is similar towards the original Undertale. Only the characters seem to be different so far.

Not much is known about Negativetale yet, since the AU is only several days old. I'm eager to see how the skelebros will be like in Negativetale, though.

Well….she tried killing you with a fake hug and ended up killing herself……

Karma can be a sweet dame at times~