Overview of Filters
Most lenses have circular threads for attaching a filter to the front. As can be seen here in this Canon 16-35 f/2.8, one would need a circular filter size of 82mm in order to screw a filter onto the front of the lens.
Now, let's say you have another lens you like to use with a 77mm filter size. Obviously, if you bought 82mm circular filters, they'd be too big for your 77mm size lens. What do you do?
A very traditional and strongly encouraged option is to get what is called a "step-up ring" for the smaller lens. This is a simple ring that has a 77mm sized thread on one size, and an 82mm (in your case) sized ring on the other - thus allowing you to use your larger circular filters on your smaller lens. Here's on example:
There are some drawbacks to doing this that are covered on this site. But, it gets a lot of bang for your buck.
Another Option: Filter Systems. Systems are designed to use square or rectangular filters, such as 100mmX100mm or 150mmX150mm. The filter holder attaches by way of the front threads and yes, one normally uses a step-up ring to bridge the gap between whatever the lens size is to what the filter system size is. For example, this Formatt-Hitech 100mm system attaches at 82mm - so any lens wanting to attach would need to use an X-82mm step-up ring where X is the filter size of the lens:
Filter systems come with some pros and cons compared to circular filters and this too, has been covered on this site.
Why you need Something Special
Let's take a look at the front of your Samyang:
Take a look at that front element! The lens has a very protruding front element and I'm betting it would be impossible to have wrapped it in plastic so as to provide front filter threads without vignetting or straight-up cropping the image. The built in hood is there as well.
What this Means: Neither normal circular filters or a regular filter system will work with your lens. Why? Because there's nothing to attach them to - there are no front threads on your lens.
So, to use filters with your lens, you have two options:
Option 1: Buy a specialty filter holder. NiSi and Haida have offerings, and I'm surprised these even exist. Why am I surprised? Because the relative market share of people looking for filters for this one lens has got to be very small. Unfortunately, this means that you need a piece of specialty hardware - which means $$. It is what it is - there's no way around this.
The good news here though, is that you will need another, regular filter system for use on other lenses - but the 150mmX150mm filters will work between both filter holders - so at least you get more use there.
Option 2: Sell the 14mm Samyang. Yea, I know, not the best option. But, to avoid having to use specialty filter holders, get a lens that has front filter threads. I don't think you'll get as wide as 14mm going this route - but it is an option.
My own thoughts on your situation: Just like one should never skimp on good tires for their car, one should also never skimp on glass. This includes filters. Unfortunately, your 14mm lens is so wide, that you need 150mm filters to cover it. (I personally use a 100mm system, but I only ever go as wide as 16mm - and even there, the system I use vignettes hard)
150mm is bigger than 100mm. This means more glass. This means higher cost per filter.
That's the tough pill you need to swallow if you want to start adding filters to your lens. And unfortunately, if you're wanting to do daytime long-exposures (and trust me, I feel you on this one) it's the cost of getting the shot. You can't do daytime LE without some big ND's - and big ND's cost, especially the 150mm ones.
So, Haida or NiSi? I have no experience with either but the Haida seems to be constructed in parts while the NiSi appears to be constructed as a single unit. This alone would lead me to paying more for the NiSi. You want the filter holder to be solid - it would be a terribly heartache if you tilted the lens down, set up a LE, and then failed to catch the filters as they tumbled off your lens and onto the ground.
The holder is the only thing keeping your sizable investment in filters attached to the lens. IMO, this is not the time to skimp for cheaper or badly made parts.