Did anyone calculate the effictivnes of both options?
yes, there are old posts saying about this.
Training and friendlies have the same effect (development - condition loss).
But there are others pros and cons.
The +4% for the man of the match , the money (if you have a big stadium), some extra boosters (a red, green and 1-2 blue) .

Cons are the injuries. You need a lot of reds to fix players.
Skills are gaining more randomly.
You should watch the games to see if it will go to the player you want and not a random one.
You must have some friends from lower levels so to secure your victories.