The choice is yours but both offer something different. Adding vertical stripes will add the illusion of height, while horizontal stripes will give the illusion of space. Once you have decided which you are going to use you need to decide on whether you are going to paint one wall to create a feature wall, or two walls or all walls in a room.
However, stripes can be overpowering if used in abundance, so we suggest that you use them sparingly. Ideally on one or two walls and combine with some smart living room ideas to create a designer finish. If you do decide to go all out try using subtle colour combinations and use artwork to break up the stripe extravaganza.