Arata gets so little screen-time and development that its obvious who its gonna be and its obvious who the author wants it to be. This romantic situation is very commonly used and its usually the one in Taichi's situation who wins, even if you disregard the screen-time and development issues I mentioned.
Sure, all Taichi got is the "Boo hoo. I'm in love with her but she's more interested in another guy" thing going for him but its still a hell of a lot more than Arata, plus it gains him major sympathy points from the fangirls. All Arata has is the sparkles that Chihiya gives off whenever she sees him, which can mean anything (this IS a shoujo). Even in shows where she's admittedly in love with the Arata, he still usually don't win in the end unless he's obviously the lead male.
Yeah, I like Arata more than Taichi in terms of character but unless there are some major changes, Taichi is the obvious intended choice and the only one that makes sense. Even if there are changes, its probably way to late to change things now.