Shortening the border would make more sense, if the politics could be taken out of the equation. Most of the illegal immigration comes from this very border. Their border to the south is very small. If they ever wanted to unify with the US, I think we ought to consider it for this very reason.
Because unifying with a population of predominantly poor, unskilled, and potentially very dependent people would be a good trade-off for having a smaller border with Guatemala.