... The more I ask, the more I'm lost, I could be someone who would buy a set of strings because of its packaging.
Luckily I don't have a favourite colours. ...
While I love the old-school sound of plain gut and plain gut-like strings, they have very fast decay (very little "sustain")... Plain guts can also be very loud, but they tend to be dark in tone If you're interested in pleasing your desire for slow decay (lots of "sustain"), plain guts or plain gut-like strings are not the strings you want. And btw, plain guts tend to be easier to bow than synthetic wrapped or synthetic wound strings (some which are nearly impossible to bow).
It sounds like you want modern sounding strings, which is exactly what Spiros are, and that's why we tend to keep mentioning them as if they are a standard --
they are. Spiros have very slow decay (lots of "sustain"). They also have strong volume and they tend to cut through the mix well, they have a slight metallic tone compared to non-metal core strings.
For me personally there are three valid reasons I do not use Spiros (or other high tension metal core, metal wound strings):
1) I have hand problems related to arthritis and my hands don't tolerate high tension.
2) I play an aluminum double bass and strive to minimize metallic tone with my strings.
3) I prefer an old-school sound.
So for my needs, plain gut or plain gut-like strings are the best answer.
If I didn't have those reasons, I'd at least have tried Spiros, and probably would be playing them today.
And btw, the reason I tend to use the terms decay and "sustain" is that technically sustain is the length of a note like in terms of having a key down on an organ keyboard or like the stroke of a violin bow -- long sustain means the organ key is held down, the bow stroke is long for the note. Decay is how long the note continues after a string is stroked, or after a drum is hit. So for pizz on a double bass, decay is the right term; for arco on a double bass, sustain is the right term.