Flying freight is definately more challanging than flying pax. You fly most of the time at the limits. Typical day:
wake up 2am - you feel like sxxt
first sector: Maximum Take-off weight. Rate of climb somewhere between a C -152 and PA-28.
Landing at maximum landing weight, wet runway.... Hiaaah
Second sector: AC almost empty. I climbs like a rocket. Great fun.
Approach in bad weather, aircraft empty, the handling just doesn't feel right and you are bloodey tired...
But as you can see the flying envelope is much wider than on pax-ops and you gain more experience from it.