Similarities and differences between the two continents there's in abundance, some of which are easier to notice and catch the eye much more.
Similarities between North and South America are that both have mountain ranges on their western parts stretching in a linear manner, in roughly north-south direction, having lowlands in the middle parts, and having highlands in close proximity tot he eastern parts. The population of both continents is steadily growing, and there's big Spanish influence in both of them, including the language and the culture.
Differences can be seen in the climate with North America being dominated by continental climate types, while South America is dominated by tropical climate types. North America's population is predominantly white while South America's is predominantly mestizo. The culture in North America is mostly influenced by England, while the one in South America by Spain.