Go with the 4" if you like a lot of muzzle flash, noise and are a masochist.

Go with the8 3/8" one if you want something for self defense. You won't even need any ammo you could just use it as a club.

Sorry couldn't resist.

But seriously either one would be a handful and both would have the same bragging rights and wow factor. Plus the ammo is is a real wallet lightener about $1.50 to $2.50 around.