Partner ACS R6 - R8 (new, of course) is a better choice than the Partner version of IP Office. It's easier to install and maintain; the voice mail options are superior to embedded messaging; it has been "burned in" for about 20 years without any major problems (yeah, yeah, I know about the initial R7 manufacturing issues, but the were identified and eliminated in a matter of months).
Look at it another way: if you install the ACS, in 12 or 15 years (or even more), the system will need replacement, and if you or your customer are still around, then you can argue about the choice then. It's likely that neither one of you will be; nor will the Avaya marketing personnel who have been pushing the IPO version; nor will the engineers who are responsible for designing either version. Telephony will be a different industry in 15 years, and it's doubtful that the desk set as we now know it will exist.
So: make it easy on yourself and your small customer and put in the ACS.