We've owned both, both have served us well. This is a Ford/Chevy debate.
Back in the day Spartans typically had IFS (independent front suspension) and Freightliner typically had a straight beam front end.
Today they both offer each.
Freightliner does have a much larger nationwide service network, although Spartan has been good reimbursing us for any warranty work done at a non Spartan service center.
As far as the engine, bigger is better, and torque is the number that is more important in the diesel world. HP sells stuff, torque is what gets the job done.