Which is better? In my opinion it's Gear Second because even though Gear Third gives Luffy a lot more physical strength, I think Gear Second offers Luffy a nice balance of enchanced strength and speed which in my opinion is perfect for combat. Also if I remember correctly when Luffy was fighting the Gorgon Sisters during the Amazon Lily Arc, he said that he was going to use his full power against them, and by full power, Luffy meant Gear Second.

Lastly, please don't choose one you think is better because you think it's cooler than the other, actually choose one because of the abilites.