Ok a very quick look has thrown up;
Usefull load 6% advantage to the Jab
Fuel Cap 12% advantage to the Jab
Speed - around a 9% advantage to the Jab at 75%
Fuel used per hour identical
Power J400 120hp, DR1050 100hp
Price - Jab £40k DR1050 £22k
Not sure which would be considered to be the best value for money, but it is much closer than I expected.
Rod1