The NP300 is rated at 3.5T as well. Here's the most important question: is the GCM of the Merc 6T just like the Navara? If so, there will be (not MAY be) some compromise between loading the ute and towing.
About the only vehicle I've seen that doesn't have that compromise is the Land Rover Discovery. The vehicle's GVM plus the ATM is equal to or less than the rated GCM (weight of the vehicle plus weight of the trailer is less than the max allowed combination mass).
Almost every ute in the Navara's class (Ranger, Colorado, DMax, BT50) has a compromise. Note that I am not considering Courier, Rodeo, Great Wall, Ssangyong, Foton or JMC) because none of them go beyond 3T towing capacity.