“ça lui fera du bien”

is just an objective way to speak. That will be good for him.

“ça ne lui fera pas de mal” has two meanings:

Nothing to worry about. He can try it and that can eventually be good to him.

or

“Seriously, he needs to do something” and for instance eating less and dietetic things (because he’s too fat or other…)

To me there’s not one stronger that the other. They have different nuances, but I wouldn’t say one in stronger no matter what context it’s in.

If you’re helping you friend go through a depression (and you bring him a tea of something) “ça te fera du bien” feels stronger, whereas if you’re angry at someone and want to teach him good manner, you might use the other one.