I'd personally focus on confirming the coach met your needs on the livability side more then Cummins vs CAT. That being said, and I don't think they came in the Inspire, I would avoid the CAT transmission (most CATS were with either the Allison 3xxx or 4xxx series, but a few of the BIG BLOCK cats in a few CC model years, had the CAT transmission coupled to them.
If you had two coaches that you liked the interior on, and one had the Cummins and the other had the CAT - I would then go with the rig that had the best DOCUMENTATION of maintenance and care.
I believe that most Inspires had the C9 Cat?
I have no worries about CAT providing life of engine/coach support of the engine on parts and service. In our coach hunting, looking at Intrigue as our preferred coach, I wanted 1) Big Block CAT engine 1st 2) Cummins ISL engine 2nd. We ended up with an Allure, once my wife saw the galley - it was game over. No CAT option on the Allure, so we have the ISL.
I've heard it commented on that CAT parts can be more expensive, but in the overall scheme of coach ownership - I would again take the coach that had the best documented care and condition. CAT vs Cummins is a non issue, IMO
If it is a 2006 era Cummins ISL400 (Not sure which Cummins was in the Inspire?), do take the serial number of the engine and call Cummins to see if it is one with the at risk "wrist pins". I personally would avoid those engines. (They may never cause a problem, but if the do you have an expensive engine repair. And, those coaches should have much lower resell going forward, due to the risk.)
Best of luck on your search,