Behind the bar I prefer syrup because it’s easier and faster to pour. At home though I’ve noticed it’s easier to just put the amount of sugar into the tin rather than batch a bunch of syrup. All of it dissolves in the tin and its not having a noticeable effect on taste.
I’ve never had issues with that, even if I just put sugar and hot water into a bottle and shake. The only thing that happened so far when I do it this way is that the sugar starts crystallizing after a while. I usually do rich simple syrup (slightly less rich when I use the “mix directly in bottle” method), though, might be different for 1:1 syrup.
It’ll eventually becomes a science experiment, and the problem with molds like that is by the time it’s visible, it’s been growing for a while.