The second one. In the first one, you can only shoot to the left and right, in the second one you can use your mouse to aim and shoot.

the 1st one has a lot more charm imo. They're both fantastic yet pretty different experiences. I honestly would like to see more of GGC1 rather than two. 1 gives off that Metal slug vibe so well I keep coming back to it over and over again.

1st one, had more charm.
I prefer these kind of games with controller. However the second forced me a bit to play keyboard & mouse. Becaus you can shoot in all directions, it helps with aiming.

both are good games though.

The second one. The First one's movement and shooting is very limited, and your speed get reduced dramatically when being hit, not to mention lots of crazy achievements(like the one that demand you get past Army Stronghold but kill no enemies).