Apple script- Safari tweaking app

[RobertD63]

All right Im making a Apple Script app that will make it to where your able to edit the safari 4 UI. Right now it looks like:

--Just getting it started
display dialog "Welcome"
-- Pick from list
choose from list {"Change the tabs", "Change the loading bar", "Change URL autocompleate", "Disable Top Sites"} with prompt "Choose one of the following things you would like to change in Apple's Safari 4 Beta" default items {"Change the tabs"} OK button name "Choose"

--DebugSafari4TabBarIsOnTop moves the tab bar back where you expect it to be
do shell script "defaults write com.apple.Safari DebugSafari4TabBarIsOnTop -bool NO"
display dialog "Thanks!"

This is just to make it look like something is going on here. But what I want to do is when the button cancel is selected it will quit the app. But how would I call that? Right now it shows as a variable
Never mind I solved this one with this code:

if {button returned:"Cancel"} then quit

Also I'm stuck here, how would I define multiple variables when using the list? I found an example script that does it once but if I want to call a variable for changing the tabs how?

-- Pick from list
set imgType to choose from list {"PNG", "JPG", "GIF", "PDF", "TIFF", "PICT"} with prompt "Pick the image format you would like your screen shots to be in." default items {"PNG"} OK button name "Pick"