The most problematic part of choosing rectangular ND/Polarizer
filters lies more on the size of the filters (not their width, most,
if not all, filters conform to Lee standard width by having foam pads
in the edges to allow a tight fit in the holder).
But if you choose filters smaller than your future wide lenses,
you’ll end having to purchase a new set.
Or on the other hand, if you go for the bigger sizes the price of the
filters, holder and adapters will increase exponentially, and you
don’t end buying that beautiful 14 mm f:1.4, you wasted the extra you
paid for them.
Yes, you can buy knockoff filter holders, but good quality filters
don’t come cheap, and if they fall from the holder for whatever
reason you will be regret the day you bought that cheap holder for
the rest of your live.
There are few major brands (like Lee, Nsi and Hitech) that make high quality filters compatible with the gold standard, the Lee system. Others, less known, also make accessories and filters compatible with the Lee system.
The most renowned brand in the industry, and the one to which everyone compares, its Lee. They got an extremely good web page that I think it will help you decide what are your current needs and allow to predict your future necessities, and show the options that will cover each.
I believe that it’ll be helpful even if you decide to go for any other brand.