Hi! If your engagement ring is a pink sapphire (congratulations by the way!), then for me, I would definitely get a spessartine. I would not want the ring to compete with the pink sapphire. The spessartine (e.g. Mandarin Garnet) is a beautiful stone. It also has a very high refractive index, so that stone will just sparkle and glow. The rubellite to me, has too much blue in it, so it looks like a dull raspberry. It is neither a bright pink sapphire nor a fire red ruby. Plus, don't fireflies glow orange yellow?! I think the spessartine would be super cute for a firefly.
I am sure either will look great. Congrats to you again!