While the handsets that precede the S6 and the M9 - Galaxy S5 and One (M8) - are very different devices when compared to each other, the new models are expected to have quite a few things in common (besides Android Lollipop). Note that we're talking about the regular Galaxy S6 and One M9 here, not about the S6 Edge and M9 Plus.
There are (at least) 5 features that both the Galaxy S6 and One M9 will most likely offer - although, of course, we'll be able to confirm them only after Samsung and HTC announce the two smartphones.
1. All-metal designs
While the One M9 will probably not be that thin, its metal unibody design should still be great to look at. We can hardly wait to put the Galaxy S6 and the One M9 next to each other to see which is prettier.
2. 64-bit, octa-core processors
The Galaxy S6 will use Samsung's own Exynos 7420, while the HTC One M9 is almost surely going to use Qualcomm's Snapdragon 810. It's too early to say which will be faster, but, once again, we can hardy wait to compare them.
3. Rear cameras of at least 16 MP
The One M9, on the other hand, is expected to bring a dramatic change in the rear camera department, replacing the 4 MP UltraPixel Duo set-up of the One (M8) with a regular 20 MP camera. Hopefully, image quality will be dramatically improved, too.
4. Advanced front-facing cameras.
As for the Galaxy S6, this, too, should feature a 5 MP wide-angle front-facing camera, possibly the same that's found on the Galaxy Note 4 (or an improved variant of it).
5. Power saving modes.
We're pretty sure that the Galaxy S6 and the One M9 (both of which might pack batteries under 3000 mAh) will also make use of intelligent power saving modes.