And the reason why you might prefer it to another Mario fan games?
My reason: Back in middle school I played Mario Forever that I didn't know why it exist in my PC (no one in my family played it). I played it casually until world 6-2 where I can't solve the maze (I didn't know Mario can enter upside down pipe), and so I stopped playing the game. Years later I visited the game again, this time I knew how to solve it by looking at walkthrough of world 6-2 on youtube, along with looking at the other later levels where I amazed how difficult looking they're (I love challenges), after that I completed the game and having fun with all the secret levels. Since that day I decided to play the NES Mario games again (I have played SMB1 since kindergarten, enjoyed the game but nothing too special), and loved SMB3 so much. Because of that I'm become a Mario fan to this day.
Mario Forever is a very special game to me, the things I like it over SMBX is that the bosses is always Bowser but with different stats and sometimes with added abilities (like bullet blaster on his hand), has the Super Mario All-Stars graphics which is the most beautiful looking 2D Mario graphics, (the remake) has a lot of levels (without downloading a bunch of fan levels), and the level maker (Mario Worker) is easier to use. The only thing prefer in SMBX is many playable characters to choose from (I love playing as Peach, but I think there should be 2 jump buttons for her: one for float and the other for normal jump, the normal jump is essential to precisely jumps on trampolines and flying koopas instead of floating above them), that's why I can't wait for Mario Forever Revolution to be completed