I've shot in both classes-apples and oranges.
I can personally be more stable with shooting sticks/bipod but i have to be sitting on the ground to do so. Those sticks reduce alot of side to side sway that is really hard to get rid of when in a shooting harness/bumbag open style. I think it probably has something to do with how much wider your "base" is with the sticks as their feet are wider than anything any normal human is going to accomplish with their legs in a typical bumbag seated position in open. The sticks/bipod also make it easier to isolate the gun from your heart rate and breathing.
But, that increased stability of getting to use a bipod is made up for by only getting to use 16x scopes.
The other major difference that everyone knows about but nobody discusses is that open class is a freaking arms race and hunter class shooters seem to usually use guns that have not been heavily modified nor were originally manufactured with the express intent of field target. The dang knee risers on open class guns can cost more than the entire setup (gun/scope/bucket/sticks/etc) that I have seen some hunter class shooters use.
One other difference is the level of shooters in each class. Don't get me wrong, there are exceptional shooters who prefer to shoot in hunter class, but, in my opinion (based on matches that I have gone to) the bulk of open class shooters are more talented shooters. As I type that I realize it sounds inflammatory. That's not my intention. I guess what I'm trying to say is that at our club, in a 6-10 person class of open shooters, almost all 6-10 participants will have hit at least 30/40 with a good number hitting 35/40 and the 1st couple places having hit 39/40 or 38/40. Looking at the same numbers in hunter class, the top 1 or 2 places in hunter are usually in the 30/40 range while the rest of the shooters are in the 20/40 or less range. I think that is an indicator of general talent rather than a harder or easier class.
"Harder" is also subjective. Hunter is harder to shoot good scores in, mostly due to the scope limitation, but it is easier to place at the top because of who you are shooting against. Open is easier to get good scores in, mostly because of the arms-race nature of the class (unlimited scope power, highly specialized guns manufactured and dedicated with the express intent of using them in FT), but harder to place in the top slots because of who you are shooting against.
So, back to your question of which is harder, open or hunter? My answer: springer.