In PowerPoint 2013 you can export the PowerPoint file content to word or a pdf file which you can then use to compare only the text changes:
choose File> Export, Create Handouts, then click
the Create Handouts button.
In the dialog box that opens, choose the Outline Only option and
click OK. Word opens with your text.
Update:
You can run this VBS script to extract the text to a text file then you can compare the two files.
It comes from the tool Beyond Compare 4 which can be downloaded as a trial edition. Download the Additional File Formats for PowerPoint files as well if you just want to compare the text changes from the tool.
' PPT_to_TXT.vbs
'
' Extracts plain text from a PowerPoint document. Requires Microsoft PowerPoint.
' Usage:
' WScript PPT_to_TXT.vbs <input file> <output file>
Option Explicit
' MsoAutomationSecurity
Const msoAutomationSecurityForceDisable = 3
' OpenTextFile iomode
Const ForAppending = 8
Dim App, AutoSec, Doc, FileSys
Set FileSys = CreateObject("Scripting.FileSystemObject")
If FileSys.FileExists(WScript.Arguments(1)) Then
FileSys.DeleteFile WScript.Arguments(1)
End If
Set App = CreateObject("Powerpoint.Application")
On Error Resume Next
App.DisplayAlerts = False
AutoSec = App.AutomationSecurity
App.AutomationSecurity = msoAutomationSecurityForceDisable
Err.Clear
Dim Comment, Shape, Slide, TgtFile
Set Doc = App.Presentations.Open(WScript.Arguments(0), True, , False)
If Err = 0 Then
Set TgtFile = FileSys.OpenTextFile(WScript.Arguments(1), ForAppending, True)
For Each Slide In Doc.Slides
For Each Shape In Slide.Shapes
If Shape.HasTextFrame Then
If Shape.TextFrame.HasText Then
TgtFile.WriteLine Shape.TextFrame.TextRange.Text
End If
End If
Next
For Each Shape In Slide.NotesPage.Shapes
If Shape.HasTextFrame Then
If Shape.TextFrame.HasText Then
TgtFile.WriteLine Shape.TextFrame.TextRange.Text
End If
End If
Next
For Each Comment In Slide.Comments
TgtFile.WriteLine Comment.Author & vbTAB & Comment.DateTime & vbTAB & Comment.Text
Next
Next
TgtFile.Close
Doc.Close
End If
App.AutomationSecurity = AutoSec
App.Quit