Native 4K support is always better than Dynamic Super Resolution (as Nvidia calls it, not sure of others) especially these days that now 4K monitors got cheaper and well optimized. Just make sure the monitor has good specs like low input lag.
Higher DSR gives effects similar to anti-aliasing on lower resolution monitors that is held back by the native resolution output due to rendering in 4K then zoom it down to the lower resolution of the monitor. It is like opening movie then resize down the video player from the edge of the window with the mouse pointer. It could look weird.
It is still nice, but later on when filtering can be used on 4K without too much performance loss, true 4K monitors can go even higher in image quality. Also since DSR is synthetic, it might not always run as desired on different settings and apps like games and desktop.
Performance wise, DSR 4K and native 4K both have the same performance impact.