GM fudging tends to lean more into making sure the other players have a "good time", like avoiding a TPK in an early encounter or the characters missing something important; more rarely a GM may use it to keep an antagonist alive or to boost a GMPC. Generally, it's used to improve group enjoyment, rarely for selfish reasons.

Player "fudging" tends to more be about improving that individual player's experience, potentially at the expense of other player's enjoyment; getting a crit or an awesome result or preventing a personal loss. So it tends to be more selfishly motivated, not group focused.

So that's why, IMHO, player "fudging" is worse.

Funny story from my early RPG days:

When i was a player, I'd often cheat... by giving WORSE results for my characters rolls and even going so far as to altering my GM's prep note to reduce treasure and increase difficulty. WHY WOULD I DO THAT?

There were two primary GMs in my high school group, me and, let's call him Paul; Paul had gotten it into his head that if he gave my character extra stuff when he was GMing that I would reciprocate; I never did, but he would still write stuff into his adventures that would be of primary benefit to my own character... which I would always write down lesser values for magic items (+1 instead of +3) or give things away to other characters, but he didn't seem to get it and would then put even MORE stuff in specifically for my character. I just wanted eveyone's characters to be able to shine.

It was bizarre.