You guys are missing ALOT:
Hunter mark is a Bonus action spell, FF is not. so you can do a bonus action that doesnt have concentration THEN cast FF on an enemy.
Hunters mark uses a spell slot, FF does not. (giving you room for a different spell while still having the dmg benefit, for example i took zephyr strike with my new gained spell space, granted using FF will cancel its concentration, but once youve moved to safety with the zephy strike benefit, might as well start blasting with extra Umph from there after)
Hunters mark costs a Component (verbal), FF does not.
Hunters mark only lasts longer as you level up, while FF you gain more dmg, by level 20 hunters mark still only does 1d6 whille FF can theoretically do 6d8. that's the difference of an additional 1 to 6 damage on a single foe, compared to an additonal 6 - 48 damage on a single foe by the 6th round.
AND
Hunters mark you must apply it to an enemy YOU CAN SEE, while favored foe, as long as you've HIT the enemy, you can cast it.
FF > HM hands down.
Favored Foe
1st-level ranger feature, which replaces the Favored Enemy feature and works with the Foe Slayer feature
When you hit a creature with an attack roll, you can call on your mystical bond with nature to mark the target as your favored enemy for 1 minute or until you lose your concentration (as if you were concentrating on a spell)
The first time on each of your turns that you hit the favored enemy and deal damage to it, including when you mark it, you can increase that damage by 1d4.
You can use this feature to mark a favored enemy a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.
This feature's extra damage increases when you reach certain levels in this class: to 1d6 at 6th level and to 1d8 at 14th level.