1503 is without a doubt an upgrade over 1602 in terms of content, but I felt it lost some of the charm 1602 had with the narrator/announcer's semi deep voice when mentioning events happening and the beautiful soundtrack composed in those rudimentary synths and samplers of the older days, though that might be my nostalgia goggles talking.
1701 and 1404 share a similar relationship as the previous two, though 1404 nailed a most interesting approach with the possibility of multicultural colonies, where the oriental settlements have their own tech tree similar to the regular occidental one we had throughout the series. Why would you want to mix both? Some islands have spices, indigo and quartz, which you require oriental buildings to produce those goods.
It can get a little confusing at first on getting started with your first oriental settlement though.