I am assuming the new engine is a "longblock" (block and heads) versus a "shortblock" (block only), so the comparison would be a fair one.
MB bottom ends are virtually bulletproof, so the used bottom end should be solid (save for extremely poor basic maintenance).
The top end should be okay if only 79K, but eventually you may have to look at valve stem seals, and the timing chain at around 150K.
I don't remember when MB switched from the single-row to the double-row timing chain, so check both motors for the presence of that. I would assume the new motor would be converted to the double row. If the used motor has the single row, you will want to convert over to the double-row setup. Add $$$ for that.
I would save $$$ and go for the used motor. There may be some ancillary components on the used motor that may be in better shape than what is left of your current engine compartment.
New motors are generally only supplied with the basic motor and internals. You have to have the rest of the connections (hoses, harness, and other fittings).
The new motor would have a limited warranty versus the used motor. But considering the relatively low mileage on the used one, I think it's a better buy...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car