Mazda engine was discontinued at the end of 2014, so the Mistubishi-based PSI engine is the new standard.
I've got about a dozen of the PSIs in service now,The first is just now hitting 5000 hrs and we're in the process of changing the timing belt.
Unlike the Mazda, these are interference engines, so timing belt maintenance is critical. And as you might expect, they are generally a little more complex to work on than the old Mazda, but nothing extreme.
Only problem we've had with them so far is an o-ring on a coolant tube that seems to fail on all of them. They'll start to weep coolant from under the exhaust manifold. Fairly easy repair where a Viton o-ring and a little copper RTV have proven to be a robust solution.
The PSI engine is a modified Mitsubishi. PSI buys the bare block and head and retools it to their standard. Not sure what the changes are but it is different than the stock engine. The Kubota is the optional engine.