Top to Bottom would result in a symmetrical cut and is the most common method.
Corner to corner is also popular, especially for chidren as it results in triangles, but additionally it provides an easier path to avoid the crust.
However, the PROPER way to cut a sandwich is not at all.
Additionally, I will further critique of your sandwich. I score it a 1 out of 5.
Negatives:
1) Your horizontal cut is not only weird, but it is also sloppy and not centered. Its further down on the left than the right, and is just a mess.
2) Your ham is hanging out over the edges. Sloppy.
3) Bottom left corner has NOTHING on it, no condiment, meat, nor toppings. This leads me to believe your entire sandwich is unbalanced.
4) The slices of bread are askew. I mean it's not THAT hard to line them up.
Positives:
1) It's got some lettuce on it. Better than no vegetables.
2) Looks to be a wheat variant, not just white bread.