I go back and forth between the two.

Tessa is undeniably cute, and a large part of that is her seiyuu, Yukana. The little squeaks she makes from time to time (especially in eps 13 of the first season) are hard to resist and when she lost her temper to Mao in that same eps endears her to you in a way that's hard to describe. The two fumoffu eps are also some of my particular favorites. I enjoy most of all the way she can throw Sosuke off balance just with her presence, it seems to be one of the few things (other than Chidori) that can shake him, and because she's his superior she can do it more effectively.

But I also like Chidori, for most of the reasons other people have already said, foremost being her personality. Chidori's personality is what can bring her over Tessa. A little confidence goes a long way.

Then again, I also found myself attracted to Kyouko during the last few eps of Fumoffu. She seems kinda awkward most of the time, but when she took her hair out of those stupid pigtails...damn.