If you use 1440P base resolution + FSR, you get a higher resolution. If you have native 1440p, and output 1440p, then you can't compare the images without downsampling the 4K FSR back to 1440p.
If you decide to measure 1440p ->4K FSR, vs 1440p outputting at 4K, then you've chosen bilinear upsampling for the native image, so in theory you could also choose TAA Upsampling for instance.
I'm not sure a proper comparison can be done.
You'll lose the frame rate advantage of comparing 1440p vs 1440pFSR.
Technically, according to these graphs, to use Ultra Quality, both consoles will likely incur about a 1ms penalty for processing.