[pjscott]

The page you linked is not actually ambiguous, though perhaps a bit tricky to read. It says:

1. If you're compiling Objective C in ARC mode, you can't use NSAutoreleasePool directly, and must instead use @autoreleasepool.

2. In manual reference counting mode you can use either NSAutoreleasePool or @autoreleasepool, but the latter has lower overhead. (This may matter if e.g. you're draining the autorelease pool on every iteration of a loop to reduce memory spikes.)

Under the hood -- at least on the version I disassembled -- NSAutoreleasePool's -init and -release methods wrap the CoreFoundation CFAutoreleasePoolPush and CFAutoreleasePoolPop functions, which in turn call the runtime's objc_autoreleasePoolPush and objc_autoreleasePoolPop functions, which are the things that @autoreleasepool will cause the compiler to emit directly.