it is totally fine to use both work and personal examples in the fit part, so far that they are correctly connected with the question.
As for your specific question, as mentioned by Vlad and Jorrit, both stories could be fine. I would choose the one which shows the biggest drive and differentiation with your peer group, while keeping the other as a backup story.
As mentioned by Vlad, be sure to use stories which are not too old, otherwise the interviewer may ask for a more recent example.
Hope this helps,