The original is amazing, but DO NOT buy the remake. There is not anything it offers that makes it worth buying over the original.
The remake made the enemies (in my opinion) more difficult, but it changed the equipment. The equipment is now so powerful, that it does not even matter that the enemies are more difficult. You're so overpowered throughout the entirety of the game that it's a breeze to get through it all.
Not only did they do this, but they limited the amount of items you could get. For every equipment in the game, you could get 2 pairs for each brother, Mario and Luigi (with the exception of minigame equipment). In this remake, however... Several pieces of gear I remember getting 2 pairs of is now limited to only 1 pair. An example of this is the D-Star Wear. You get 1 pair from the Dark Star, and one from a secret question mark block. They removed the 2nd pair.
There was once a "Challenge Medal" in the original that made the game highly more difficult. In the remake, although it's still called a Challenge Medal, it does the exact opposite. It basically makes your damage output do 10 times as much that it normally does. To the absolute most difficult boss in this game, having this item equipped, I did over 5 thousand damage. Not okay.