I compared the two a little while ago;
The Jagdpanzer IV has 230 armor compared to the SU-85's 140.
The Jagdpanzer IV has a target size of 17 compared to the SU-85's 22. This means it is easier to hit the SU-85.
The Jagdpanzer IV has more accuracy; particularly at long range. (JP4: .04 vs SU-85: .025; almost double at long range)
Both tanks have comparable speeds and penetration. JPIV avg penetration: 185, SU-85: 190
The Jagdpanzer 4 reload: 4.3-5.2 vs SU-85: 3.8-4.2. SU-85 fires about ~19% faster.
Jagdpanzer 4 is far better but it is more expensive. Jagdpanzer 4 has some really great veterancy too:
Jagdpanzer IV:
Vet1: Ability (Camouflage Ability)
Vet2: +10% armor, +160 HP, +35% sight radius
Vet3: +20% rotation, +20% max speed, -10% reload
Vet4: -25% reload, +30% accuracy
Vet5: Gains "First Strike" bonus when attacking from camouflage
... as you can see the Vet2 bonus for the Jagdpanzer is crazy good. +35% sight radius is basically permanent SU-85 vision cone with no mobility disadvantage. +160 health gives it 800 health; the same as the Panther. It's basically one extra tank shot before death which is real nice.
For the sake of balance I'll snag the SU-85's vet bonuses:
Vet 1: Tracking Ability (Increased vision range)
Vet 2 -30% Reload +30% Accuracy
Vet 3 -20% Reload +20% Max Speed +20% Acceleration +20% Rotation
... Personally I find the SU-85 rather lacking at the moment as its armor is so low that only auto-cannon units like the Ostwind will fail to penetrate it reliably. Its range is a boon but range is essentially map dependent.
Jagdpanzer 4 is pretty excellent although I don't use it much. I get a lot more out of the Puma which is just a little more flexible of a unit and arrives at a more favorable timing. It's a great unit to have in one's toolbox though as a dedicated tank hunter late game.