The difference between Life and Death and Swift sparrow?

To be Doubled during enemy's turn and the defenses penalty

Life and Death is more of an "all-in" option since it cripples defences in exchange for being active all the time. It can sometimes work to your advantage by preventing a double that might have occured otherwise but for the most part the unit you give it to is generally going to explode if they get hit.

Swift Sparrow only works on offence but you can get away without any defensive penalty due to that limit. It's generally considered the better option if the unit is going to see retaliation on occasion.

Ok lets get the most obvious out of the way.
LnD gives +5 to atk and spd but gives -5 to def and res
Swift sparrow gives +4 to atk and spd but only when the unit initiates.

Swift sparrow is better for arena scoring but LnD is cheaper.(since it is a 4* now)