Portent vs. Lucky

[Motorskills]

Portent lets you replace enemy d20 rolls too, not just your own............. That's insanely powerful. Use low Portent rolls to make an enemy fail a crucial saving throw, prevent a critical hit against you, or flat out miss an attack, etc. I'd go for Portent.

You don't know the hit will be a critical in advance. The best you can do is guarantee an enemy attack that does have Advantage won't be a critical.
Conversely, the occasional 20 can be used to guarantee a crit for, say, a paladin to spike that crazee damage!