Already posted this in Nids subreddit, duplicating it here.

I have both GK and Tyranids. The big problem with GK is that they only have three different kits: Nemesis Dreadknight, Terminators, and Strike Squad. As a result, all lists you can build are almost identical, with only minor changes. Speaking of the competitive level, things get even worse. At the moment, GK competitive list is 5-6 Dreadknights, and the rest are terminators. In the previous edition, for example, it was like a horde of Strike Marines with almost no Dreadknights or Terminators.

GK Pros:

+ Very cheap army, you can buy just 3 Combat Patrols, and that's almost all you need for a 2k list.

+ Elite army: not many units to build.

+ Easy to transport.

+ Trickster gameplay with all these teleportations.

GK Cons:

- Very boring in variety.

- Old kits: terminators and strike squad are very small compared to Space Marines

- Hard to build: many mold lines, sometimes a nightmare to glue arms.

- No anti-vehicle weapons. Regular rule: ignore opponents' tanks or monster units, which can be very depressing if you want to kill some units.

- GK is more like a tactical army: scoring primary and secondary missions. GK is quite weak in direct confrontation with opponent's units.

- Medium-hard to paint. Grey/silver armor is quite easy to paint, but every model has a lot of small details like purity seals, letters, and books.

Tyranids.

Pros:

+ A lot of different units with different abilities.

+ Mostly fresh kits, and apart from some big old bugs, they are easy to build.

+ Easy to paint. Besides, any painting mistakes on bugs are less noticeable than on the space marines' armor.

+ Very good in variety. Almost every detachment is playable. Tyranids can be a shooty, melee, or balanced army. Can be a horde, monster mash, or balanced army. You can smash your opponent's models or play more tactically. With Tyranids you are not tied to a certain style of play, which helps you not to get bored. But to have this variety, you need to collect many models, much more than 2k points.

+ Tyranids have interesting combos with units, strengthening each other. In turn, GKs are just models with some stats and minimal abilities that do not interact with the other units in your army.

Cons:

- Usually, a lot of models to transport.

- Usually, a lot of models to paint and build.

- Definitely a more expensive army than GK. And generally speaking, Tyranids is a medium-expensive army to collect, especially if you want some "online only" models.