How good a substitute is callaloo for spinach?
I've only had red amaranth, so apologies if that is distinct from the type you have access to. As I recall, spinach is a bit sweeter and the leaves are a bit softer so they break down more readily. Also I believe the stem on spinach is less fibrous.

That said, they are both leafy greens and I've yet to find a recipe so touchy that one leafy green can't be substituted for another.