Both Apps offer the same functionality, however they offer choice in terms of purchasing features as follows:
If you download the free version and then upgrade you get all the functionality but no additional PDF themes. You can then either use the default theme, choose to purchase themes individually or all the themes by purchasing the theme pack.
If you purchase the paid version, then you get ALL the functionality AND ALL the themes. If you want all the themes, then this one represents better value for money, otherwise the upgrade path is better.
If you decide to purchase the paid version after having downloaded the free version, then we would recommend you delete the free version from your device to prevent any confusion.