Originally Posted by Drip

Speed affects how fast your teams move around the map. When one team has a habit of reaching the finishing pole before the last pests are killed, then that means it gets hard to 3-medal many maps. Usually, squads with comparable speeds (like, the fastest squad is at most 100 speed faster than the slowest) are easier to clear maps, but occasionally, a map requires extreme differences in speed to get those 3 medals.
A 100 speed difference between two girls actually doesn't have a really big effect on the squad though, the squads' speed is the avarage speed of the entire squad, so if you gave Dianthus and Victoria Bride the same team members, the difference in speed between the two squads would only be about 20 speed. Sometimes that's just the bit that makes the difference, other times it doesn't make a difference.

Still whether you pick Victoria Bride or Dianthus: they're both very good units, each with her own advantages. If you prefer one over the other, for whatever reason: get her.

Whether to use a synergetic squad of 5* girls or a rainbow girl depends a lot on how well you manage to make your squad synergetic, and what synergy you pick. Or, you could even make a synergetic squad of 4, and add Dianthus (or whatever rainbow you may have) on top of that. Rainbow girls don't suffer from being in a squad with a different synergy, though some might severely underperform in such a situation. Dianthus and Victoria Bitter aren't the type of girls who are in the underperforming category. They could both actually greatly improve the survivability of your team (though you'd need to put the taunt skill on Victoria Bride to accomplish that).

and you can learn speed+ and speed- ability for the group for next to nothing