I'm going to put a different spin on this

I was disappointed not to be able to book an UD seat when I was rebooked from a cancelled LCY service - chose 20A on the LD which is right at the back of the CW cabin and has direct aisle access (when I chose this there were only 4 or 5 seats that I could have chosen)

The UD on that particular flight was full but the back cabin on the LD had at most 8 seats taken - service was good and the whole cabin was very quiet

So I think it depends on the loads - in the event of full loads on both UD and LD I'd probably choose the UD but as the UD fills up quicker than the LD then if there's a chance that the LD is not full I'd go for that