Not much of a difference. Points to be considered is:
1. Which versions are you supporting, COM add-ins are supported only on
Office 2000 and onwards.
2. Do you want to protect your code? If yes, move to COM add-ins. PPA can
make it a little difficult but the source is hardly secure.
3. Is this going to be used in an enterprise environment with strict
policies/user restrictions in place? If yes, then I use a combination of the
two, the PPA as the loader for creating menu item etc and the rest of the
code in a COM dll.
PPA code will execute faster than from a COM add-in.