What bus engine is better - Cummins or CAT?

REALLY need to know a lot more about the bus(es) you're looking to buy. Huge difference between a school bus and a charter bus, after all, and substantial differences between engine models and model years - especially in regards to diesel emissions regulations for the years they were manufactured. A good rule of thumb is to avoid Cat engines from 2005 - 2009; the 2008 and 2009 model years, especially - those were disasters which ultimately led to Cat making an exit from the highway diesel market. 2003 was a year of growing pains for Cummins. If you're looking at something with a 5.9 Cummins, they were problematic for losing their lift pumps in the years prior to them going to common rail, and, if the problem wasn't caught, could lead to the fuel pump going out, and being a much more costly repair.

Perhaps you should email me with more details of these buses you're looking at, and a more specific answer to your question can be given. Anyone who artbitraily tells you "Cat" or "Cummins" without knowing any of the details I spoke of should not be trusted.