This toggles the "Show message preview" checkbox:
tell application "System Preferences"
reveal pane id "com.apple.preference.notifications"
end tell
tell application "System Events" to tell window 1 of process "System Preferences"
repeat with r in rows of table 1 of scroll area 1
if name of UI element 1 of r is "Messages" then
set selected of r to true
exit repeat
end if
end repeat
click checkbox "Show message preview" of group 1
end tell