Idefix

7/10 Good photo, subject is isolated from background with good bokeh, but the subject itself looks too dark.

You would take a Titan X for example, and take away CUDA cores and memory, thus making a 980Ti, that's an over simplified explanation, but holds true.

Can you show data?

This is like asking "What fruit should you eat?" 
 
You're still asking a good question, asked by a lot of beginners who often do not understand "photography" because they're caught up in the "technical" aspects of the gear.
 
There are no rules that define what the proper the uses for your lens are.  If you understand how different focal lengths can alter the composition, you will be able to work the scene and decide which lens works.
 
 
I recommend you find yourself a copy of this book and read it:
http://www.amazon.com/Within-Frame-Journey-Photographic-Vision/dp/0321605020/ref=sr_1_4?ie=UTF8&qid=1440881914&sr=8-4&keywords=david+duchemin
 
Alternatively there are some nice low cost eBooks here that cover the creative aspect of photography and a bit more.
http://craftandvision.com/
 
And despite all this, you still need a rule or guideline about lenses:
Think about the composition before you choose the lens.

Ok there are some basic stuff you need to know about using lenses:
On lenses that have image stabilisation (Nikon calls it Vibration Reduction, VR), if your camera is mounted on a tripod or any stable surface they recommend not using VR. On lenses which have 3 different VR modes (Off, Normal, Active), you should stick to using Normal for most shots. Active is mainly for things like you are panning the camera and lens hand held to get some sense of motion. Every beginner seems to think opening the aperture seems to be the best substitute for low light.  This is not accurate.  As aperture controls the DOF, use it to control DOF and not compensate for low light.  Not enough light?  Well you should know how to use speedlights and strobes and light modifying tools. Want to make the street lamps in your photo appear to have starburst patterns?  Use a narrower aperture, like f/8 or smaller. Aperture and Focal Length affects the Hyperfocal Distance. Want to take close up portraits with minimal distortion?  Use focal lengths between 80-135mm.  That doesn't mean you can't use super telephotos or wide angle lenses, you just have to know the sort of image distortion they have on the subject and if that effect is what you require. http://petapixel.com/2016/02/15/why-you-shouldnt-count-out-wide-angle-lenses-for-portraits/   
 
As I said previously, think about the composition before picking a lens.  Below is an example of a photo where I used an ultra wide angle lens, my Nikon 14-24 @14mm.  It's a photo of some interns at one of my offices.  Why did I choose to use such a wide focal length?  Because the distortion effect of the lens would go nicely with the shape of the guitar, the long neck.

 
Here's one where I used a telephoto lens, and waited until the Sun was close to the horizon.  The best time to take Sun set/rise and Moon set/rise photos are when they are close to the horizon, using a telephoto lens and having some nice foreground objects.  That's how you make them appear large in the photo.  If I used a wide angle lens to compose the image below, the Sun would not appear as large in the image.
 
Here is one where I used the wider end of my medium zoom to give a sense of distance and vastness, even though I was just a couple of meters away from the front most person.

 
Use the lens and camera position to alter perceptions.  You think the image below was taken in some sea or ocean?  I took it in the middle of a lake, from another boat.

 
Or where I enjoy playing with miniature cities and towns.   I used a tilt-shift lens here.

 
Know that doubling or reducing the shutter speed or ISO or aperture changes the exposure by 1-stop each time.  ISO 200 is 1 stop more sensitive than ISO 100.  F/2 is 1 stop brighter than f/2.8.  30 seconds shutter speed is 1 stop more exposure than 15 seconds.
 
 
And in the end, as you just said...besides my experience with it is what counts.
 

Don't quote me on this, but I think a dutch startup company is making a modular phone - Fairphone. Its only modular, but its kinda like a build-your own as everything is seperate.