Both are used in QTP.

In most situations, you should use a SystemUtil.Run 
statement to run applications or to open files in their 
default application. A SystemUtil.Run statement is 
automatically added to your test when you run an 
application from the Start menu or the Run dialog box while 
recording a test. 
  Sy:SystemUtil.Run "C:\type.txt", "", "", ""

InvokeApplication statement is supported primarily for 
backward compatibility. 
          Sy: InvokeApplication "E:\Program Files\Plus!
\Microsoft Internet\IEXPLORE.EXE"