While there are a huge number of different downpipes available in terms of colour and size, there are really only two types that you need to consider. Downpipes are usually either square or round, and ultimately, it’s more of an aesthetic choice than anything else.
Square downpipes are designed to fit flush against the wall of your home, affixed by special clips that match the downpipe’s size. Round downpipes are essentially the same, held against the wall with designated clips. The only real difference here is that in some older homes, clips aren’t used for round downpipes.
As mentioned, it’s really an aesthetic choice, so you’ve got the freedom to match downpipes to your guttering and the general look of your home.