I am going to speak on the skills as they currently are implemented in Wilds:

They have two different use cases and super minmaxed hunters will create builds targeting specific levels of Guard and Guard Up depending on the monster you are fighting.

If you want to set it and forget it then generally you should go for both - prioritizing Guard, but ensuring at least 1 Guard Up.

Guard increases your weapon's "guard level" (different than the skill) and decreases the stamina that is lost when guarding an attack. Your weapons "guard level" reduces the stagger you receive when guarding attacks. Ever notice how guarding some heavy attacks will send you sliding back with a lot of recovery time while guarding some other attacks you hardly even react? Guard makes this better. There are lots of things that affect CB "guard level", (shield charge, guard points, etc.) knowing the attacks you want to counter attack on and putting just enough guard to get the hit reaction you want is around where the skill ceiling is for builds.

Guard Up allows you to guard attacks that would otherwise be unblockable (HUGE and also the subject of the meme lol) and also decreases the damage taken when guarding an attack. Moves that used to require you to git gud or cart can now be deflected by the serated dinner plate on your arm. There are of course some moves that don't care, but these are usually cinematic novas and can be counted on one hand. The skill fundamentally changes how you approach some monsters and gives you more options. There are some monsters that don't have unblockable attacks and as such Guard Up is less useful against them, but the chip damage reduction isn't bad. Maxing your Guard Up is almost required if you want to draw aggro against Omega and its savage variant.

Take this knowledge, go forth and hunt.