Surprisingly the battery in the HTC One M8S has had a bit of a boost over the M8, which may explain why the phone is slightly thicker. The M8S has a 2840mAh juice pack (the same size as the HTC One M9) while the M8 has a 2600mAh one.
That might lead to better life than the 1.5-2 days you'll typically get from the HTC One M8, but it's hard to say for sure given the M8S also has a different processor.
With up to 32GB of built in storage the HTC One M8 has double that of the strictly 16GB HTC One M8S. Don't panic though, because if you feel like storing your entire movie collection on the phone you still can, as both handsets also have a microSD card slot with support for cards of up to 128GB.
There's nothing really to choose when it comes to extra features. Both phones run HTC's slick Sense 6.0 interface (though notably not Sense 7) and both have BlinkFeed and a duo camera, albeit with different megapixel counts.
Both handsets also have HTC's impressive BoomSound speakers, so you can get the party started without a proper sound system, you know, if you like really lame parties.
This could be the big reason to consider the HTC One M8S over the standard M8, or not, depending on how it pans out.
See the HTC One M8S has an RRP of £379.99 SIM-free in the UK, which is substantially cheaper than the £549.99 HTC is asking for the One M8.
But no-one in their right mind is paying HTC's asking price, as you can get the One M8 for around £375 elsewhere. Now it's likely that other stores will quickly undercut HTC's asking price for the One M8S too, but whether they'll undercut it by much remains to be seen.
After all the One M8 is a year old, so it's understandable that it's available at a discounted price. The One M8S on the other hand is brand new, so despite being less powerful it might not be much if any less expensive.
There's not a huge amount of difference between the HTC One M8 and the HTC One M8S, but surprisingly some of what differences there are potentially work out in the M8S's favour. Its battery for example is bigger, though it remains to be seen whether that translates into better life.
It also has a higher megapixel count on its camera, though at the expense of the larger UltraPixels found in the HTC One M8.
On the other hand it has less built in storage and while it has a newer processor with more cores it's a processor that's been designed for midrange phones, so the quad-core Snapdragon 801 in the HTC One M8 should still outperform it.
Other than that the two handsets are much the same, with the same premium build and large 1080p screens.
In theory the HTC One M8S is cheaper, which could make it a very strong buy, but given the price cuts the HTC One M8 has had that might not pan out in practice, in which case it will be a harder sell. Though the larger battery and 13MP camera could still win it some fans.