Which one should I report as total network I/O bytes among Sum(InBound) or Sum(InBound + OutBound)?
I would suggest reporting the sum of both.

As a general principle, the best solution is to define your metrics in the report or paper where you write up your results.  You should describe what you measured and how you measured it.  This includes clarifying whether "total network I/O" includes both inbound+outbound traffic or only inbound traffic.