Assuming they're from the same underlying master, I think blu-rays look better than UHDs on a device that isn't HDR capable. But there are a number of 4K discs that have superior transfers to their blu-ray counterparts, and those will obviously look better. And there are a handful of 4K discs...
The player has to convert HDR to SDR and some do this better than others. It's also going to depend on how the HDR disc has been graded. I had a Panasonic player connected to my plasma and found it mostly did a very good job, though I'd say the average picture level is a little bit darker...