Actually....

The skill disadvantage is actually pretty big if you have grapplers or trippers in your party, as those require skill checks, not saves.

Hexing + grappling isn't quite as strong as you might think it should be because the target always has the option to switch to Dexterity (Acrobatics) instead of Strength (Athletics). Even if Dex is the weaker stat, it can be better than Str with disadvantage.

Don't get me wrong, Hex is absolutely worthwhile, I'm just pointing out a countermeasure.

Other uses for Hex include Hexing sleeping dragons' Wisdom (Perception) to make them easier to sneak up on, and Hexing the target of an illusion to penalize his believe check (Intelligence (Investigation)). Typically works better with a teammate than solo. Edit: also, initiative is a Dexterity check, so if you're using Speed Factor initiative or similar, Hexing Dexterity can be pretty advantageous in forcing the enemy to react to events too slowly.

Other differences:

Bestow Curse can incapacitate a target.
Bestow Curse can be made semi-permanent.
Bestow Curse V doesn't take concentration. That's huge.
Bestow Curse can be used to enhance spells like Planar Binding to make them more efficient (by disadvantaging the Charisma check).

The only real overlap I see between the spells is the "+d8 necrotic damage per attack" option of Bestow Curse, which is actually the weakest option IMO.

It's a fun spell. One of my players love to have his PC use that spell in various other ways, from giving guards diarrhea so he can sneak past them, to shutting up another PC after the second or third interruption during delicate negotiations (I ruled it as a Wisdom save in order to be able to speak[1]).

[1] The targeted player didn't seem to take offense for some reason. Maybe he knew his PC was out of line.

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