OK, here are the results of the comparison. This was very tricky and time-consuming to collate.
I took the data from the YT comments, from the thread on UU and from the thread on Cosmos.
The strings on the instrument with the tuner were regular Nylgut strings. The set on the other uke were Bionylons. 77% of people who made a guess were correct. 23% of people got them the wrong way round. However, only about half of those that made a comment tried to guess which was which.
Only about 12 preferred the Bionylon strings (9.5%)
At least 72 preferred the Nylgut strings (56.5%)
At least 43 either did not express a preference or like both e.g. one for strumnming, one for picking (34%)
Comments about the Bionylons (numbers in brackets indicate if more than one person commented in a similar way)
Mellower (9), Warmer (6), Muted (5), Dull (2), Dead (2), Softer (2), Less attack(2)
Other comments: Fuller, More Percussive, Clearer, More Vintage, Cleaner, Fat, Jazz Tone, Sweeter, Drier, Nice, Louder, Plinky, Less Tight, Subtle, Muddy, Muddled.
Comments about the Nylguts (numbers in brackets indicate if more than one person commented in a similar way)
Brighter (19), Louder (19), Fuller tone (13), Richer (7), Warmer (6), Cleaner (5), Mellower (5), Deeper (4), Lively (2), Punchy (2), Crisper (2), Brash (2), Clearer (2), More Sustain (2)
Other comments: Responsive, Punchier, Clangier, Fatter, Better Note Separation, Better Attack, Grittier, Spikier, Harsher, Brash, Forward
Please make comments and observations. Isn't it interesting that both types of string are considered mellower and warmer?