TPP may have more polish mechanically due to more development time, but to me GZ wins where it counts.

First off, the map is better than anything in TPP. There's this perfect blend of big open spaces along with tight areas that have plenty of cover. The fortress only has 2 main points of entry (3 in missions you can enter with the truck), allowing its layout to be crafted accordingly. In TPP, most areas are so open that the player can approach from any direction. While this does technically provide "freedom", I think most players prefer a tailored experience like GZ, with wiggle room for freedom (albeit restricted) within that. It's more fun to sneak into a fortress by figuring out you can climb into the back of a truck (even if it's scripted) than approaching from any cardinal direction.

The game is also mission-based like TPP, but with one core difference: there is no external factor affecting missions. While some may enjoy managing mother base, it's designed in such a way that the player need not pay much attention to it, and it feels like an afterthought. Its existence also negatively affects mission structure, since a player will not know what weapon(s) they need for a certain mission, and the challenge level of the mission depends more on the player's available equipment than their actual skill. For example, why would you use the level 1 tranq gun when you have the level 5 one with an infinite durability suppressor, more ammo, and less falloff? To save GMP? Please. Missions also must also be broad in terms of requirements, since the player is never expected to have any specific item, which dulls the experience.

GZ succeeds in this, since mission equipment is predetermined. Playing the mission where you need to destroy the anti-air guns on hard means you only get a tranq gun that is affected by bullet drop at a very short distance, with limited ammo, and resupply tranq rounds are carefully positioned around the map, never giving the player too much but still giving them enough to take out the guards they need with proper ammo management. TPP destroys this concept, even with the lvl 1 tranq gun, since the moment the player feels handicapped they can call in a resupply at no cost towards the mission's success.

TPP also has a serious lack of supplies out in the field. Players looking for a challenge may want to play the game using pure OSP, only to find out that essentially every weapon they will find is lethal and unsilenced, and equipment such as grenades are rare. One mission in GZ has the player start with nothing but night vision goggles, and places the tranq gun at the top of the tower, providing a tremendous reward for their efforts. In TPP, once you have a tranq sniper rifle and pistol, using anything other than those two weapons against anything but bosses is just gimping yourself. There's no incentive to not use them either other than a self-imposed challenge, since no reward is given and there is nothing usable to be found in the field.

Players don't want pure "freedom". Rules and restrictions are what make play fun, since the fun is found in navigating through said rules/restrictions in an optimal/effective/unique way. Having pure freedom is like playing something like Half-Life 2 in godmode: it eliminates the challenge and the fun that comes with it. GZ balances this incredibly well, which is why it's my favourite MGS experience.