The white nylons have been my default string for fretless since they were released. I love their tone, sustain, tension and chunky gauge (I use the standard, heavier, gauge). The copper whites were lovely too, but they've stopped making them in extra long which I need for my fretless (strung through body, see pic below), so I am just using whites.
I also have a set of medium scales on my fretted Epiphone Masterbilt.
The black nylons are great if you want the same feel but with a darker tone.
They may be expensive but they last years, so it all evens out in the end (as long as you like them of course).