24, 72 or 144 hours?

Hi there!

My partner and I are flying from Melbourne to Seattle via Shanghai. She's American and I'm Chilean so both seem to be eligible for any of the free transit options.

Our layover is only 18 hours long. Do you recommend the 24, 72 or 144 hour free transit (to be on the safe side?). Any downsides to applying for 144 hours and only staying for 18 hours?