How much of a difference in IQ of the nifty-fifty against the EOS M 15-45mm kit lens?
I personally have found the kit lens sharp and image looks a bit modern (or not? compared to EF 100-300mm I owned). The price is also almost the same as the 50mm or even can be higher. Another positive thing about the kit lens is that it is much more recent (modern).
Some photos I took with kit lens.
Using the-digital-picture.com's test data comparing the two lenses side by side with the EF-M 15-45 set to 45mm and wide open at f/6.3, and the EF 50mm f/1.8 STM (on crop) set to f/5.6, here's the center performance:
EF 50mm f/1.8:
EF-M 15-45 f/3.5-6.3 IS STM:
So, yes, there's a noticeable image quality difference. But you are kind of comparing apples against oranges here. The 15-45 and the 50/1.8 are both built to be cheap and are consumer grade, but one's a walkaround zoom that's kitted with the camera body, and it goes from wide to short telephoto. The other is a fast portrait prime that can open up to f/1.8 and can be more easily used in low light or to achieve thin depth-of-field. They're two different tools and better suited to different tasks.
This is more of a theoretical question that likely needs to be tested.
But, in theory, a prime lens will always be sharper than a zoom lens.
In theory, a kit lens balances cost over performance, intended for most people to deliver a better shot than they got with their 'other' camera. Most kit lenses are noticeably less sharp than other lenses in a typical range.
In theory, the nifty fifty will be sharper than the M lens, because aberation is more severe at the edges of a lens, and the EOS-M would not use the edges of the full frame nifty fifty, but instead use the more sharper center of the lens.
In theory, the EOS- M kit lens image stabilization and STM for superior use as a video lens offers more modern features that the nifty fifty, which has no IS and may be using the older focus motor. On the other hand, theoretically, it should be far easier to handhold a 50mm lens for less blur, such that IS is not that significant for most shooters.
May be not fully answer to the question, but shortcommings of EOS M 15-45mm can be corrected by Canon DPP, giving fully acceptable photo quality.
