You would be better off buying them off the website, so that you can make some fast pass selections, before your arrival. You can make fast passes 60 days ahead, if staying in site and 30 days off site.
FYI: For anyone looking at DLR tickets, note that 3+ day tickets purchased at the ticket booths do not come with a Magic Morning. So if anyone is trying to decide between the booths or buying ahead online/on the app -- skip the booths. There may not be a financial savings in buying tickets on the DLR website or on the DLR app vs. at the ticket booths, but there certainly is a savings in time and convenience.