OFV or Diagnostic Plot ?? Which one rules...

Hi Everyone, I am fitting two compartment PK model to Marijuana (THC) concentrations. When I apply proportional error (or proportional plus additive) residual model, I get pretty good fits (except 15% of subjects) at all time points. However, when I apply only additive error residual model, I get perfect fits in all subjects but objective functional value is increased by about 20 units. DV vs IPRED reveal all concentrations on line of unity. My question is: should I go with additive error model which gives me perfect fit but higher OFV or should I go with proportional error model which gives me lower OFV but not so good fit in couple of subjects? Regards, Sumeet Singla Graduate Student Dpt. of Pharmaceutics & Translational Therapeutics College of Pharmacy- University of Iowa