Have you played all three or four versions?
What are the differences between k1 plus, k1 and the European version?
Is K2 difficult to control because it is the hardest one?
I have played with K1 and K1 plus extensively. I can read Chinese so I know a lot about K1 Pro and K2.
You can look at the following comparison:
Sponge hardness: K1<K1 plus<
K1 Pro<K2
Topsheet tackiness: K2< All K1
Ease of engaging the sponge: K2<K1 plus <
K1 Pro < K1
Because K1 pro uses a different sponge structure the sponge is easier to engage than K1 Plus despite the hardness.
K2 is a beast with reduced spin and control.
K1 Pro is the most popular in all the variants in China.
K1 Euro is basically K1 with reduced tack on the topsheet.