I think it depends on the content and how you intend on reading it.

Pdf is best for content that has very particular formatting and graphics but because it's preformatted, it's not so good for reading on small screens

Epub is best for mostly text content that doesn't have too many graphics since the text can be formatted on the fly for your screen, so it's good for phones and small e-readers

Generally I stick with epub for most books but will get pdf if it's like a textbook or something more technical