I don't think it has much to do with learning reading or writing first, but more about learning letters before you progress to words, before you progress to sentences, etc.
I don't think it really matters if you start with writing or reading first, because it's basically the same skill: knowing letter shapes and knowing which sound corresponds. It's that learning really benefits from receiving the same information in different contexts. So as long as you both teach them to write and read, the order doesn't really matter.
One reason for using the cardboard letters for younger children is that the brain regions handling bodily movement are much more advanced, and thus provide more context which (as stated earlier) helps in memorization.
But I suspect the main reason for using the sandpaper letters is because it is way more interesting for the children to have something with tactile feedback, instead of just looking at a flat card.
I don't think it really matters if you start with writing or reading first, because it's basically the same skill: knowing letter shapes and knowing which sound corresponds. It's that learning really benefits from receiving the same information in different contexts. So as long as you both teach them to write and read, the order doesn't really matter.
One reason for using the cardboard letters for younger children is that the brain regions handling bodily movement are much more advanced, and thus provide more context which (as stated earlier) helps in memorization.
But I suspect the main reason for using the sandpaper letters is because it is way more interesting for the children to have something with tactile feedback, instead of just looking at a flat card.