"for one's benefit" means for one's own sake, for the advantage and good of someone. " in one's interest" means to one's advantage,as a benefit to oneself. So they are almost the same and could be interchanged.

Both mean pretty much the same thing

I think it all depends on how you use them. If I saw them as is, I would say they don't mean the same thing, but are similar. "For one's (own) benefit" means, well, for the benefit of one's self. And "in one's (best) interest" something that would be in the best interest of that person. "It would be in your best interest to study for the test tomorrow." If your teacher gives you extra time to study before the test, he might say "this is for your own benefit to get warmed up before the test. Use your time wisely." Did that help at all? If not, I apologize, and I'm sure someone else will be along to answer soon.