I agree with the other posters that it does feel early to be voting for characters right now, but I can't resist voicing my opinion. My feelings may change when more episodes come out.
Epiosdes 1-4 I liked Mion and Rena equally and as a pair. Rena with her cute tendenices, and Oyashiro-mode dialogue and look. Mion with her way of taking charge and friendliness. The two made a fun combo to watch.
Episode 5 tipped Mion in her favor for me. She's like Asa-sempai in Shuffle or Tina Foster (voice by the same seiyuu as Mion) in Ai Yori Aoishi, two other characters who I also like.
Mion showed a nice range of emotions. From the outwardly strong, extroverted, take charge girl we knew in Ep. 1-4; to Ep.5 where we see her more vulnerable and sensitive side, yearning to have be more feminine and having it that side appreciated by Keiichi.
Like KJ said, Mion is the girl you would love to have as a friend. Kind of why I like Asa-sempai and Tina Foster.
Shion is the intriguing counterpoint to Mion. It is great to have a character like her to play off Mion to bring the best and worst out of each other. If there was vote for supporting-actor category, I would vote for Shion. (Not to actually suggest starting a supporting-actor poll, just my hypothetical musing).
Okay, it's time for me to stop drinking the Mion-Shion Kool-Aid.