I think that both Rukia and Orihime are important to Ichigo. If I have to choose one it would be Orihime.
The way it may seems, Rukia may be important to him is because she helped Ichigo to achieve the one thing he always wanted to do in his life. As it said in one of the episodes, Ichigo wanted more from his life, he wanted to be able to protect his friends. Rukia helped him with that.
The reason why he went back to Soul Society was to repay her. Plus, she was his friend.
This is cheesy but I think the bond they have right now seems like love. However, it will turn into a Harry Potter/Hermoine type deal where they seem like they should be together, but they become friends in the end. Rukia seems more like the sister type.
Plus, with their personalties, I think Rukia helps him to find his true self. She is around him way more than Orihime.
Anyway, It may also seem like the bond of love is deeper between Rukia and Ichigo because they fight a lot together. Rukia does get hurt often and he is there to save his friend.
Now to Orihime. Ichigo does not spend as much time with her. They have been childhood friends and the hint of a chemistry has been there since the beginning.
He didn't go save her in Heuco Mundo because he owed her something. He did it because of there friendship (plus fighting).
I look at Orihime as shy and modest. I bet there is way way more to her then what the show leads on right now. She is not at all boring or annoying.
She may not have the greatest power (yet Aizen believed she was like a God). She will though. She did have the power of words on Ichigo... It helped him to beat the Espada #6.
Anyway, I feel that if Ichigo understood love and the feelings that Orihime had they would be together. I am soooo rooting for Orihime and Ichigo pairing.
I believe the writer of Bleach has left himself a lot of room to develop Orihime's character!!! I can't wait to see what he has in store for her!!
I never want that friendships between Rukia and Ichigo to change.
The quiet ones suprise people the most!!!