Unless the dark mode is completely 100% black and not what 99% of dark modes merely are which is simply just an overall darker GUI (often grey or blue) then you are not really saving any battery as the pixel is illuminated regardless.
Regarding OLED and LCD.
There are benefits to LCD on the way sub pixels are arranged over OLED.
Let's take two QHD Screens, of which a 1440P has a total of 3,686,400 Pixels.
Now when we count how each pixel is arranged, the subpixel arrangement.
On a 1440P OLED screen there are 3,686,400 Green SubPixels, 1,843,200 Red Subpixels, 1,843,200 Blue Subpixels.
This means there are 7,372,800 lighting elements in a 1440P OLED Screen.
An IPS LCD has 3 subpixels per pixel.
This means 11,059,200 lighting elements on a 1440P LCD screen
This means a 1440P OLED Screen only has 67% of the same number of tiny little colored dots which make up detail of a LCD screen of the same resolution.
1080P has 2,073,600 Pixels
OLED has 2,073,600 Green SubPixels, 1,036,800 Red Subpixels, 1,036,800 Blue Subpixels.
There are 4,147200 Lighting Elements on a 1080P OLED Screen
Once again IPS LCD has 3 subpixels per pixel. This means there are 6,220,800 Lighting Elements on a 1080P LCD Screen
So what does this all mean?
A) A 1440p LCD has 11M subpixels, which is 4M more subpixel lighting elements compared to the OLED despite them sharing the same resolution. A 1440P pentile oled (7.3M lighting elements) in terms of image quality, is therefore closer to a 1080P LCD screen (6.2M lighting elements) than 1440p LCD screen.
B) a 1440P OLED screen used in the hand, you might not be able to see the pentile, but in VR where the pixels are magnified, it is still VERY relevant, an IPS 1440P screen has 11M lighting elements where a pentile oled only has 7.3M, therefore an IPS screen will give you much more detail.
Of course there are definite benefits to OLED such as contrast & black levels but currently no 100% percent answer to one being better than the other. A lot of it at the moment boils down to subjective preferences.
The future will change that.
However, where there will be a very definite improvement is when full RGB AMOLED displays finally launch. Where we finally have the benefits of both technologies without the weakness. However it isn't looking like we are closer to market of those screens just yet, despite them 'on the way' for the last number of years. (Super Amoled Plus is closest we have right now).
The take from all this is that there is still room to improve with regards to our displays in the future. That future will be Full RGB AMOLED. Samsung's Super Amoled + tech is getting ever closer to that realisation, hopefully 2018 will finally usher the true next gen.