Both reports show the pages of a site analysed with social metrics.
It's just in Top Content report we are trying to keep only pages like blog, news articles and similar content pages, but Pages - Best by shares report comprises all pages we crawled from a domain.
Besides, it is to be noted that social metrics can only be exported from Top Content report.
Thus, if you are interested in the number of all social signals your site receives on the whole, it would be advisable to check Pages - Best by shares report (since it includes all pages of your site).
But please note that social metrics in this report are only available on site with no option to export them.