Team Mitsuki, all the way. She was there for Takayuki when he needed someone (not that Haruka could help it, I know). Also, you know from the "Next Season" sequel (and the ending montage of Rumbling Hearts itself) that Haruka will do just fine: she finds someone else, publishes her children's books, etc. Mitsuki, on the other hand, is completely lost without her athletic career and without Takayuki.
There's another sequel called "Akane Maniax" that, unlike "Next Season", is compatible with the Rumbling Hearts timeline (ie, Takayuki ends up with Mitsuki). It's actually set up for "Muv-Luv", the visual novel that Ã¢ge did after Rumbling Hearts, set at the same school, and it's arguably more a Muv-Luv prequel than a Rumbling Hearts sequel, but it's interesting how it meshes the two worlds.
Also, if you're coming to Anime Central 2015 in Chicago, I'm doing a panel called "Rumbling Hearts to Total Eclipse" that covers the entire history of Ã¢ge.