So, I've recently invested in a 16-35 II. I'm looking to use an ND filter for long exposures, but I'm having trouble settling on one. [...] I've just seen a NiSi filter on amazon. 10 stop coated filter in 82mm for £49.98.
I've recently researched this (and ended up with a Heliopan 10x for €100), so here it goes:
* for cloud blur in daylight 10x isn't enough, get a 16x, also helps for removing moving objects and improves iq since you don't need to use the smallest f
* cheap filters have worse color casts, just don't.
* you don't need a premium-coated version because a nd is there to stop light (you might get more flare though)
* most important thing: both b+w or heliopan 10x produce color casts due to infrared leakage, if you want to avoid this (it's not that easy to remove in post) get a filter with integrated hot mirror ir blocker - it costs more, but with wider lenses than 35mm you cannot stack an ir blocker with the nd filter.
Indeed the heliopan 10X produces significant color cast