Topographically, it makes very little difference; you've got the same amount of ascent and descent either way.
Important considerations might be whether you live in the north or the south and whether you like your house and spouse/partner or not, i.e. whether you would be happier walking towards home or away from it - not that this is any help if you live in East Anglia or across the pond.
Best of all would be to walk it both ways as (a) being twice the distance there's twice the enjoyment, (b) you can cut down on what you carry, leaving dirty clothes at accomodation on the way out and collecting them on the way back, and (c) you can save a bit on the train fare, buying a return to either Presatyn or Chepstow rather than two singles.
Hope this helps, and have a good trip.