The pixel density is 166 PPI.
This value is confirmed by several reliable sources for a 27-inch screen in 4K UHD (3840 x 2160 pixels), although some mentions vary slightly around 163-166 PPI.
I was able to do some research and according to the manufacturer's specifications you are correct. The difference is in the Smart TV area and audio. The rest should be the same. I was also unable to find out which arguments were ultimately decisive in the creation of this model. However, according to the specifications, it is a very interesting monitor. The "G81SF monitor" is this year's model and the other is from last year. According to the specs, I can well imagine the argument that you don't need smart TV and audio, but prefer a better panel and a better picture.
Yes, it has a 3.5mm jack port, on which you can use headphones or speakers when activating sound with an HDMI or DP cable plugged from your PC to the monitor.