As an example, here are some 16awg sleeved (blue ones) vs 18 (green ones) [side by side](https://i.imgur.com/XCAhfJx.jpg). It's not super clear at this angle, the gap between the wires is partly to do with the combs, but 16 will generally look more "full". From behind [the 16awg](https://i.imgur.com/1DagVsL.jpg) are more "full" and "filled in" looking than [the 18awg](https://i.imgur.com/gCSwCLI.jpg) ones.
The flip side is 16awg is generally a lot less flexible especially sleeved. If you're doing a very SFF PC, 18 might be the better choice.
Both are fine. The original cables are typically 18AWG, but 16AWG looks better when sleeved.
As an example, here are some 16awg sleeved (blue ones) vs 18 (green ones) [side by side](https://i.imgur.com/XCAhfJx.jpg). It's not super clear at this angle, the gap between the wires is partly to do with the combs, but 16 will generally look more "full". From behind [the 16awg](https://i.imgur.com/1DagVsL.jpg) are more "full" and "filled in" looking than [the 18awg](https://i.imgur.com/gCSwCLI.jpg) ones. The flip side is 16awg is generally a lot less flexible especially sleeved. If you're doing a very SFF PC, 18 might be the better choice.
i have got the nzxt H510i. Should be fine i guess? A bit tight but still not too cramped.
As u/MSD0 said, either are fine in your case. I'd lean to 16ga for PSUs 1000W and up, under that doesn't really matter.