Technically either. You're best bet would be to take them into your local shop and see which size binding fits them best. I think you'd get away with the medium, but it may be a little on the snug side.
Never Summer SL153 & Atomic Hatchet 159
I would say go larger rather than smaller. If the straps are a little long it's probably easier to shorten them. On a smaller size, if you've already adjusted them to the max and they're still to short then there's probably not much you can do.
I also live in TX. Sun and Ski is in most larger TX cities and carries a decent selection of bindings around this time.