The "Mac Box Set" includes three DVDs: Snow Leopard, iWork, and iLife '09. The US$29 DVD of Snow Leopard is the same, but without the other application suites.
If your Mac is currently running Tiger (OS 10.4.x), you cannot upgrade directly no matter which DVD you use. The Snow Leopard installer will refuse to install over Tiger. You will have three choices:
[1] Erase the entire hard drive.
[2] Create a new partition to install Snow Leopard separate from Tiger. This has the advantage of allowing you to boot to your old Tiger system to use older software. To do this, you have to either boot to target disk mode, connect your Mac to another Firewire Mac that is running OS 10.5.x or 10.6.x, or boot to an OS 10.5.x or 10.6.x Install DVD.
[3] (if your Mac has Firewire and you have enough available space on the hard drive) you can make a folder called "Previous system", boot to target disk mode, connect your Mac to another Firewire Mac move the Tiger folders (Applications, Library, System, and Users) into the "Previous Systems" folder, and then boot to the Snow Leopard DVD to install normally. With a non-Firewire Mac, you could do this in single user mode (UNIX command line), but it's not for the faint of heart or those with bad typing skills, as you can mess things up by typing incorrectly.