Didn't mean any offence about 2003, it was directed at the company's IT staff / CIO / etc., I apologize if it came across the wrong way.
The whole "Exchange in AquaMail" thing has been dragging for too long (and I apologize for that too), it's high time to get it done.
Now, on EAS vs EWS.
Using EAS requires a license from MS. I looked into it, and even though I'm not at liberty to disclose the details, in brief, the licensing scheme is too complicated to be practical for my app.
This let me with having to choose between OWA and EWS, and I chose the more modern EWS.