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You get a 1 deg field with either 32mm/50, 24/68 or 40/43 eyepiece and as you say the higher exit pupil is desirable for the horse head but so is a very dark sky. You don’t necessarily need the wider field as 1 deg. is more than enough.
It comes down to how much you want to spend and what other targets you may want to observe with h-beta with the wider 2” eyepiece. Astronomik filters are very good, not familiar with the Kson brand.
I went through this a few years ago (not specifically h-beta) and invested in 1.25” filters of various brands until I found my preference and then invested in 2” of that brand. What surprised me was the variation between brands of the same type of filter but overall all impressed. I have Tele Vue (1st gen, no h-beta), Lumicon, Astronomik, Thousand Oaks.
The horse head when you finally see it is black on black and not as the show piece that other nebular are such as Eta Carina with 2” O-III or veil nebular with the same and of course M42 in all three filters.
I’d get the astronomik 1.25” and see if you like H-beta on other targets too. M42 looks different with H-beta. I use 1.25” filters with 1.25” eyepieces and 2” filters with 2”eyepieces. Seems obvious but.... some people buy 2” only and screw onto the bottom of their 2”-1,25” adapter. I DO NOT recommend this approach as you risk damaging your 2” filter if your eyepiece has a long barrel or you’ve left a filter on a 1.25” eyepiece. Even Tele Vue don’t provide for this option as they deliberately do not thread their 2”-1.25” adapters and if it’s good enough for them it’s good enough for me.
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