Best way to structure data, tabular or matrix?

I hope this isn't a dumb question, but as a new Power BI user I've been struggling with the best way to structure my data? Namely is it better to store things in a more tabular or matrix format. For example if I want to do analysis on a series of stocks, lets say Microsoft and Apple. Is it better to structure data such that:

Tabular:

Date, Return, Stock

1/1,  10%, Microsoft

1/2,  -5%, Microsoft

1/1,  3%, Apple

1/2,  -7%, Apple

or 

Matrix:

Date, Microsoft Return, Apple Return

1/1, 10%, 3%

1/2. -5%, -7%

I know this is quite generic and probably matters a lot on the use case/scalability issues, but I wasn't sure if one of these was generally better. I imagine a compromise would be to store the origin data in a tabular sql table and when importing it into power bi to create several queries filtering the original data set and then linking together.

Hopefully I'm not completly ignorant on this and the question makes sense! Thanks.