The Jagdpanzer IV has better self-spotting (45 frontal sight range vs 35);
The Jagdpanzer IV has way higher DPM (5s base reload vs 6.55s reload);
The Jagdpanzer IV has a way better target size (17 vs 24);
The Jagdpanzer IV has better stationary accuracy (0.06/0.05/0.04 vs 0.05/0.045/0.035);
The Jagdpanzer IV has better durability at vet 2 (800hp vs 640hp);
The Jagdpanzer IV has a better veterancy accuracy bonus (x1.38 vs x1.3)
The Jagdpanzer IV has better veterancy mobility bonuses (+20% rotation and speed vs none, which gives it higher top speed than the Jackson);
The Jagdpanzer IV still has much higher DPM with veterancy (3.5s vs 5s);
The Jagdpanzer IV has camouflage at vet 1 to always get the first shot off.
The Jackson has a turret;
The Jackson has better stock mobility (6.5 speed, 3 acc and 30 rotation vs 5.5/1.9/20);
The Jackson has better accuracy while moving (0.75 moving accuracy vs 0.5).
In conclusion: in general, the Jagdpanzer IV wipes the floor with the Jackson.
JP4 plus King Tiger is the most potent combination. I see Duffman use this combo almost every OKW game and it works wonders, especially vs Jackson spam and UKF.
King Tiger is probably the worst match up for Brits as someone mentioned earlier. I personally think King Tiger is in good place and the tank feels balanced. The problem is a lot of people use this unit in a wrong way and completely forget combined arms. The tank is not a YOLO dive type tank.