As an orthodox full-back, neither at the moment.
Hard to weigh up. Somner is the more solid defender, but can't offer much going forward, lacks pace and doesn't always have an instinct for where to position himself when playing left back. Fieldwick contributes more going forward, but shows his inexperience at the back sometimes.
I'd go 3-5-2 so you can have both of them playing to their strengths rather than exposing their weaknesses, but realistically I think we're going to have to hope Frampton is fit soon.