Seems like you have two options.
Have the user select the field value from your dropdown list. Then have them press a "Go" or "Continue" or whatever button. That button triggers a script that checks the field value and brings them to the appropriate layout.
Set the field in question to not be user-editable. Instead, have one button for each option (in your example, two buttons). Clicking on one runs a script that enters that value into the field and then goes to the correct layout.