SUN still has a long way to go with releasing content. I temporarily quit when the level cap was only 50. Dekaron/2Moons, on the other hand, has more content.
You don't drop anything in 2Moons if you PK and are killed.
There are hackers in 2Moons. Lots of them. The new server Rondow is probably the hardest hit with the hackers atm since there are no patrolling GMs. Being a PVP/PK game, levels make a difference and hackers are clearly in the advantage.
2Moons definitely has more grinding than SUN, since they released 100% and 50% EXP cash shop items and lowered the EXP curve twice already.
In the end, if you enjoy a more secure game, I'd suggest sticking to SUN. If you don't mind hackers ruining the game however, you can try out 2Moons.