A lot of people hate the Wii Zapper and love the HotD Hand Cannon. That's primarely because the Wii Zapper is a strange thing that doesn't really look like a gun at all. It's also really expensive; especially in Europe (30 euros, about 45 US dollars).
I really love it. You can use both the Nunchuk and the B button on the Wii Remote. What makes it so good is that you can aim really precise. If you stretch your arms and use your left hand (or whichever is closest to you) to kind of pull the Nunchuk backwards you can hold it really still and aim fast and precise.
An obvious downside is that the face buttons of the Wii Remote (Dpad, A, -, +, 1 and 2) aren't available for use, and you can't seperately move the Nunchuk from the Wiimote either (so you can't jump by moving the Nunchuk upward without moving your cursor).
In the end, I'd go for the Wii Zapper if you want to use it only for games that specifically support it, like House of the Dead: Overkill, Medal of Honor: Heroes 2 and Call of Duty: World at War. If you just want a nice cheap shell that should work with as much games as possible, you should get either the Hand Cannon or another similar gun.
