If you were disappointed by the limited updates that HTC brought to the HTC One M9 (and it looks like you'll probably be disappointed by the camera if nothing else) the M8S might be a more interesting phone for you. Mixing some upgrades and downgrades, this might actually be the most attractive One yet.
The M8S carries over the design, screen and features of the outgoing M8, but improves the Duo camera (13mp) and adds a bigger battery. As a tradeoff, you'll drop down to a weaker (though still octacore) processor.
Seems like a reasonable swap to me.
The M8S will also be a bit cheaper than the top of the range M9, with UK prices of around £380, compared to over £500 for the flagship.