Which is better: a stiff or flexible snowboard?

It completely depends on your style and where you're riding! Stiff boards give you more control and stability at high speeds, perfect if you're hitting challenging terrain or want clean carving, but they're less forgiving if you don't have the technique. Flexible ones are more versatile, they forgive mistakes and are great for park, powder riding or if you're a beginner, though you lose a bit of stability at top speeds. I personally prefer something in between because I spend a lot of time in the mountains looking for those brutal landscapes for photos, so I need good board control without sacrificing comfort on varied terrain.

What a lot of people don't mention is that flexibility also changes depending on where it's concentrated on the board: a board can be rigid at the ends but flexible in the middle, which gives you the best of both worlds for park tricks without losing stability on open terrain. If you're a beginner, I'd go for something with medium flexibility because it's easier to control and less punishing if you make mistakes, but once you know what kind of riding appeals to you, that's when you should look for a board that suits that.

Stiffness also wears differently depending on how you skate: if you're learning or doing tricks in the park, a flexible board is gonna forgive you way more for balance mistakes and impacts, while the stiff ones need precision. My trick is that you try one out on a rental first before dropping cash, because your weight and how you land matters way more than what the spec says on the box.