It sounds like you're actually talking about downscaling, not upscaling. If your resolution scale is >100%, you're rendering at a higher resolution and then downsampling to 768p. It is essentially super sampling anti aliasing, and comes with a similar performance hit.
To answer your question, native 1080p will still look better than 1080p downscaled to 768p. But is should look better than 768p at 100% render scale.
It'll be about the same. The image is being rendered at 1080p either way, and the downsampling that's currently being done to output 768p should have little to no impact on performance.
Edit: Assuming that your scaled resolution is exactly 1080p. In your case, 1080p is actually ~40% more pixels in each dimension, or ~2x the total pixels. I can't seem to find a reliable answer as to whether resolution scale is referring to the total pixel count or the pixel count per dimension.
It depends on how the application is defining render scale. 1080p is nearly double the number of pixels as 768p. So if "150%" resolution scale means 1.5x the pixels, you're still well short of 1080p. However, if "150%" means 1.5x the vertical resolution and 1.5x the horizontal resolution, you're at 1152p and therefore above 1080p. As I said above, I can't find any definite answer on which is the case.
If you're rendering at 2049x1152 with your current render scale, I'd expect native 1080p to perform a bit better.
Edit: What graphics card do you have? If you enable VSR (AMD) or DSR (Nvidia) in your driver settings you should be able to have an actual 1080p option you can select in games for resolution. Rather than trying to approximate it through resolution scale. Both options do pretty much the same thing though (render at a higher resolution then downsample).
I have RX 580 8GB, i cant have VSR because i have an active converter from HDMI to VGA (monitor has only vga) and the converter cant handle it, when i had it cinnected directly to my older GPU it worked fine, now it says !OUT OF RANGE! Everytime i choose something higher than 768p