Now as to parameters for comparison, one could use the "steepness" in the center of the fitted plot, i.e. find where the first derivative is maximum and compare maximum first derivative values. This idea comes from the fact that the perfect knowledge model has "infinite" steepness at the 100 g mass.

Input := 

Plot[u'[t],{t,0, 200}]

Output =

-Graphics-

Input := 

sol = FindRoot[u''[t]==0,{t,100}]

Output =

{t -> 99.2788}

Input := 

u'[t]/.sol[[1]]

Output =

0.0100408