Both of these movies serve as prequels to their respective series by introducing the main characters and the main conflict. Both are epic space operas that focus on an intergalactic war between two opposing sides. They also have main characters who share similar goals.
These two films hold similarities in being the beginnings to extremely long and complex stories that span lightyears and hundreds of characters. While LoGH aims for the more realistic depiction of war, FFS has a more flamboyant style and story, but remains just as broad in scope and detail. Both films also are relatively contemporary to each other, also both are more or less an hour long.
Both Bubblegum Crisis OVA and LOTGH: My Conquest is the Sea Of Stars are not required to watch the sequels (BGC Tokyo 2040 and LOTGH OVA Series), but they help you to understand better the storylines. In case of Legend... it's even better just because the anime turns to be more enjoyable if you gather all info possible upfront.