Late to the thread here but I use filters 90% of the time and would advise you to take the hit now (if you can) and get premium filters, rather than buy cheap and “upgrade” in the future.
I use a mix of Lee 100m and Formatt Hitech Firecrest filters. I have two Lee holders, one with 3 slots and another with Polariser ring at the front and 2 slots behind. I really like the Lee holder. I looked at Nisi and F-H Firecrest holders but felt they were, well - a bit “faffy” for want of a better phrase. I don’t want to be faffing around with polariser wheels, light seals and steppingrings on location, so I stayed with the quick, reliable Lee holder.
As for the filters themselves. I think Lee have lost some QA on their grads. Given they are hand dipped resin, there may be slight variances, although they do get tested to tolerances (wide or narrow tolerance?). My older ND grad filters manufactured in 2013 are a dream, very, very neutral with the right level of gradient. The soft set from 0.3 to 0.9 are indeed soft. The 0.6 hard is hard.
What bothered me is when I added a 0.9 hard to my set last year (November 2016 manufacture date) it was verging on a very hard transition and appeared to throw a magenta/ orangey red cast onto some images. I complained and sent it back. Lee were good to be fair and tested it and said it fitted within their specs and tolerances. They offered a replacement (manufactured January 2017) and whilst the gradient was closer to the hard of my 0.6, it still has a cast. I gave up at this point. I use it sparingly and might just try and replace it - especially if F-H Firecrest come in with hard grads in the future. This might be unlikely, you’d think they’d have been able to do it by now. I might look at a Nisi 0.9 for the time being.
Onto solid ND’s. I used a 0.6 Lee ProGlass and found it to be accurate and neutral enough, but the Firecrests that replaced it are definitely better.
I loved my Big Stopper, but couldn’t use it with film. The famous blue cast caused some nice effects at times, but when you see and experience a completely neutral shot from a Firecrest, it has you wishing you sold the Big Stopper earlier and converted sooner!
Polariser, I nearly opted for a Lee landscape polariser, reasonably priced, fitted my system but it’s got a warm cast (no thanks), stopped more light and isn’t multicoated. Deal breaker.
I thought about a Heliopan Slim, multicoated. Ridiculous price and hard to find. Filter thickness may have caused vignetting.
I finally opted for the Ultraslim Firecrest CPL. 1 stop light loss, multicoated, no cast, no vignetting, fits Lee CPL ring and although expensive - I had a coupon code for 30% off from an ambassador. Deal done!
Anyway, sorry for waffling on! Hopefully someone finds this helpful!