This is certainly a hard question. I used a single plus carrier for children 1 and 2 (21 months apart). But I bought a double planning for child 2 and 3 to go in it (18 months apart). Turns out Child 2 wouldn't sit in a stroller if Child 1 was walking, so it barely gets any use! But it has been handy on some occasions.

I paid $400 for my City Mini Double 2nd hand and if I had my time again I wouldn't have got one or would have bought one cheaper than that.