Let the mass of the light body = m kg ,  its velocity = v m/s.
Let the mass of the HEAVY body = M kg,   M > m , its velocity V m/s.

    KE = (mv)²/ (2m)  =  (MV)²/(2M )   
             (mv)²/(MV)² = m/M   < 1        =>   mv < MV
 The HEAVIER body has higher momentum.
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2)  lighter body:  m kg and  velocity V m/s
     heavier body :  2 m  and  velocity V/2 m/s
       Their momentum is same.
   But    KE :  1/2 m V²   :  1/2 (2m) (V/2)²
             KE :    1 :  1/2 
      The lighter body has a higher KE
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3) 
    DRW trucks are used for very heavy load carrying trucks.  For lighter loads, they are not really advantageous.  The additional wheels add some weight on the rear suspension.  Then the springs on the rear suspension have to be stiffer (higher spring constant), to handle them.

   The area between the tyres is LESSER as compared to the SRW trucks.  So if there are bumps or if the road is rougher, then the truck dances more.  There is more stress on the axle parts and the body.Cost of the wheels and the related maintenance is added.For very heavy trucks, the load is evenly distributed on tyres.  So the pressure is reduced.  The trucks can carry higher loads.  For very long and higher trucks, is important.  SRW may be able to control the sideways pull (sway).