I tend to be a sneaky bastard when games allow me, so naturally i'm gonna say melee. but gunplay has it's uses when meleeing.
I see a camp of Marauders ahead of me. I realize this is the gang the *inserttotallyuselesslocallawenforcementagencyhere* has ordered me to execute. I sneak into the camp, Silently executing them with my knife. as I work my way from building to building, i'm noticed by one of the sentries, and soon an alarm is ringing throughout the camp. I whip out my Uzi and start mowin' people down.
and then there's an example like this. I'm wandering the Wastelands when a raider appears on the horizon. I quickly relieve him of his gun by shooting it with my blackhawk. we then engage in hand-to-gauntlet.
in short, Melee when possible, guns when melee fails or helps melee.