Though Instagram is the better choice on a smartphone, when I search for, say, "asparagus recipes" in Pinterest, the results are so much better, more relevant, and more aesthetically pleasing.
If I search for the same thing on Instagram, it shows me the most popular posts, which are good, but after delving into them are not as good as what Pinterest spits out at me.
Also, when sifting through the results of a search on Instagram, you have to click into a post if you want to save it. It's not the most arduous thing in the world, but Pinterest makes it nice and easy to remain on a search page and to save a post to a board without having to navigate away.
Instagram also doesn't allow URLs in posts, so it's more of a drag to redirect from a food post to the linked website containing the full recipe. Meanwhile in Pinterest, you can immediately click into the recipe from a post if you want. You can also refine your searches heavily in Pinterest, like selecting a 15-min prep time when searching for dinner recipes.
And lastly, I've noticed the Pinterest algorithm has gotten more and more intuitive, recognizing which board I'm wanting to save a post to based on the nature of the post i.e. an outfit, a recipe, a workout.
I'm sure Instagram will catch up with that at some point, but for now, my collections seem to only pop up in one order whenever I'm deciding which one to save a post to.