I haven't tinkered much with ND filters, but I thought it was high time to start learning how to use them for land- and city-scapes, perhaps a bit of video as well (then again, I've said I'd do more video for years now).
I would pick up a variable ND filter to make it more convenient and user-friendly. I'm more of a travel and documentary photographer.
So, if I was about to start and only wanted to buy one VND to learn and grow with, should I go for 6-9 or 2.5 stops?
2-5 seems good enough for a novice, but when I started looking around, I noticed that many people use a minimum of 6 and often 10 stops.
You also might want to consider the NiSi True Color Swift VND which is a 1-5 and you can add a 4 stop ND to the front with a quick snap in place to make it a 5-9 VND. Reviews are good with no X cross pattern and neutral colour.
My impression is if you're shooting stills and going for the super smooth long exposure look, especially in direct sunlight, then it'll be more than 5 stops. You can run into problems with infrared contamination with high opacity NDs if they don't effectively block IR as well as they do visible spectrum. Higher density VNDs might have color shift/consistency problems.
I'm only starting to look into VNDs myself, more so for video work. What I have done so far I've generally managed to get by with a fixed 3-stop ND, which indicates to me I'll be happy with a 1-5 stop VND most of the time. Except maybe if wanting a really shallow DOF/wide open look in sunlight which probably needs around 7 or more stops depending on shutter speed, ISO and maximum aperture. The Nisi True Color 1-5 stop VND seems to get good reviews and their Swift option mentioned above might be a good one for going beyond 5 stops. But there are a lot of options from a lot of new and unfamiliar brands. Almost the only way to know reliably if one will work for you is to actually buy and try it.
The use cases for a 2-5 and 6-9 are quite different, and only you can answer that. For me, a 2-5 would be the play, as I usually just grab my 4 for tripod work and midday long exposures. Sometimes, I want a 6, but could easily make 5 work.
You can also just take a bunch of pictures and mean average in photoshop for anything you need past 5.
