SAE vs Metric, Whats actually being used on these things?

My 99 Detroit has plenty metric

 

Working on mid 90s Jeep driveline once needed a metric half size socket.That was a new one on me

 

Buy extra 10mm's and you're set.

 

Now consider when you have a modern tractor and a modern Utility reefer. The tractor will be mostly metric, the reefer unit will be mostly metric, but your Hendrickson suspension and body of the Utility will be SAE. No serious repair person can escape having sets for both standards. The exception is when you get to larger fastener sizes; minute differences between the standards aren't enough to compromise the fit of a 33mm socket versus a 1-5/16" socket, for example.

 

Like others, I recommend having a good set of both.
I do find that for most things I tend to be able to get away with using mostly metric providing things aren't overly rusted or stripped to begin with.