for my toddlers, i would always put them across the aisle. it's much easier to assist them that way, than to have to walk around to the other aisle to reach them. for the 787, the bassinet is at the window so you would have to put the toddler across the aisle anyway. for the 350, the middle seats have bassinets but as mentioned, it is harder to assist your toddler reaching over the central divider.