Roundnet is the sport's official name; Spikeball is the brand of its best-known set maker — roughly like Kleenex and tissues.
The distinction is not pedantry, it is practical: anything to do with rules, leagues and clubs runs under the name roundnet. The world governing body is the International Roundnet Federation, and in Germany Roundnet Germany runs the competition structure with 42 clubs and communities. Search for “Spikeball” and you find sets to buy; search for “roundnet” and you find people to play with and tournaments near you. The other way round, “Spikeball” is perfectly fine in everyday use — nobody says roundnet on the beach, and the two words get used interchangeably. Only rule questions are worth the switch: whether a ball may touch the rim, or how big the no-hit zone is, lives in the IRF rulebook — and that book only knows roundnet.