It's way worse mechanically.
1dx once a turn vs 1d6 on every hit
There are situations where Favored Foe is mechanically better and there are situations when Hunters Mark is mechanically better and in some situations you would not want to use either. In general, I think Favored Foe is better in more situations than HM.
On 2 attacks with HM you are talking about 5 points of damage average. Your bonus action is often worth more than that damage even without measuring it against FF. Not always but often. If you are concentrating on HM you are going to be using your bonus to cast/assign it most rounds. Not every round, but most rounds it is going to gobble up that bonus action.
Favored foe at 1st level is 1d4 per hit, every single hit you use it on, with no bonus action use and no restriction on splitting the damage between multiple enemies if one enemy goes down or you hit on a reaction attack or something else. 1d4 is on average about 4 damage on 2 attacks. More often than not, saving your bonus action is worth the 1 point of extra damage you would do that round with Hunters Mark.
This is especially true since Favored Foe damage can be stacked with other Ranger bonus action stuff like Zephyr Strike or Ashardalon's stride as long as you use them sequentially.
The fact that HM requires a bonus action to move or mark a specific foe also works against team play. HM is best when you can leave it on one enemy for an extended period. From a team perspective focused fire is generally best and it is generally best specifically BECAUSE enemies die quicker, making the Ranger move HM and wasting your bonus action on more turns.
I've used Hunter's Mark in play back before I realized how awful it was and VERY often you are concentrating on it and getting no damage at all out of it because of the bonus action cost.
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