The Macho Brace and the Power Items do not give out the same EV output. The Macho Brace doubles the ammount of EVs gained. Say a Pokemon that gives out 2 EVs, is defeated while holding a Macho Brace, you'll be gaining 4 EVs in that stat instead. The Power Items add a total of 4 extra EVs in a stat when used. Meaning if you defeat said Pokemon, but hold the Power Items instead, you'll get a total of 6 EVs, due to them adding 4 more EVs in the end. Basically:
Macho Brace: Doubles EVs of the Pokemon you're facing
Power Items: Adds 4 EVs in the end after you defeat a Pokemon
