The 6.4 Apache and 6.4 Big Gas are different engines.
From a power standpoint, the 6.4 isn't better than the 5.7. However, the 6.4 Big Gas is a ground up HD engine, not just an engine adapted from other platforms. The MDS is the major weakness of the engine but baring MDS defects the engine is pretty incredible (forged bottom end, iron block, massive flowing heads, sodium filled valves, etc). I think at some point I'm going to pull my 6.4 and build it for FI, it really only needs pistons, cam, and headstuds to reliably make 800hp on boost.