Flying with baby dilemma, PE in lap or economy own seat?

Please help out a stressed mom with a dilemma. I am flying with my husband and baby this coming week on a transpacific flight, 14 hours. Currently we are booked on two premium economy seats, with 9 month old baby in lap. We priced it out and can also downgrade to 3 economy seats for about the same price. We definitely can’t afford Polaris upgrade. So the question is what’s more comfortable, benefits of premium economy but baby in lap or extra seat in economy? We haven’t flown premium eco but the increased space sounds like sanity saver for such a long flight.