difference between x.atomicOp!"+="(1) and atomicFetchAdd(x, 1)?

[mw]

Hi,

Anyone can help explain what is the difference between 
x.atomicOp!"+="(1) and atomicFetchAdd(x, 1)?

 From the doc, their return values are different

atomicOp
Performs the binary operation 'op' on val using 'mod' as the 
modifier.
Returns:
The result of the operation.


atomicFetchAdd
Atomically adds mod to the value referenced by val and returns 
the value val held previously. This operation is both lock-free 
and atomic.
Returns:
The value held previously by val.


Apart from this, any other difference, esp in a multithreaded 
program? Are they the same? Is atomicOp also lock-free?

Thanks.