I also think that both are important. I do not have the DC-6 but looking at the screenshots above I can only agree with the poor quality there. Keeping in mind that the DC-6 was an aircraft from the 50s/60’s mostly built by hand, without the CAD and laser cutting tools of today, those textures are still looking below par.
Now, first of all, I have to say this. I do not spend a penny on commercial third-party aircraft. Never did for FS9, FSX and never will for MSFS. Why? Two reasons: One, when I look through the files and textures (as I do) it seems that I always find something even the modest me can do better. Two, because right now there is a big shortage of large airliner aircrafts in the sim and commercial devs are obviously trying to capitalize on that. I believe that with time, freeware third-party developer groups will form and these guys and gals will then push the boundaries of what is possible to create in the sim. This is what happened with FSX over the years with well known groups such as TDS, OpenSky, Project Airbus and so on. It is also important to point out that the MSFS SDK is still a WIP.
Now about the way to achieve good looking graphical results, I believe, depends on 2 things. One is a good 3D model and two is good quality textures. I have been making liveries for years in FSX (DX10) and I have learned a few things. For example, you cannot add specular (or bump) textures to an aircraft unless the model actually supports it in the first place. So the model is pivotal for a good result.
Then, like it has been said already above, the texture resolution also makes a difference, 8K, 4K, … bigger is better of course, but bigger will also affect your FPS, as bigger textures take longer to load. So here I think a compromise is needed, where excellent quality texturing should be achieved at the lowest resolution possible. That is what I usually go by in my projects.
Here is below a little screenshot of what I mean. I recently made an A330-900neo conversion from an FSX model. Now there are 2 important things here: One, the model has been modified to take advantage of the MSFS graphic engine. Two, the textures are 2048x2048px (HD/2K) only, with Alpha transparency, and saved as DXT5 using the FSX SDK! Now I hope you will agree on the quality of the final result.