Dental fillings

Lucky enough to have an NHS dental appointment today, but it revealed a gap between old filling and tooth in one of my back teeth. Option - NHS for about £60 for a new filling (metal) or £150 for a private white filling which the dentist said will last for longer. At the back of my mouth, it won't really show, but apparently the white filling can be moulded round tooth to protect it further. I would like to preserve teeth as long as possible. Anyone had experience of this? Any advice or recommendation please? I don't want to waste money, but can afford to pay for the private filling if it would be beneficial.