I've found lots of offers for Office Pro 2013 Plus. Here is a Microsoft info page on that version:
http://office.microsoft.com/en-us/professional-plus/ The difference is in the content of the applications, the "Plus" version adds Plus, Access, Lync, and InfoPath to the Office Professional
package.
I'm not sure you need the Plus version especially if your need deals primarily with using PowerPivot with Excel. Basically all of the versions of Excel are (pretty much) the same. They are definitely the same in Office Pro 2013 Excel and Office Pro Plus
2013.
However, the Plus tout page does offer this information about Excel in it:
"Scan spreadsheets for errors, hidden info, broken links, and inconsistencies using Inquire in Excel. View audit trail of changes in spreadsheets with Spreadsheet Compare."