A little dish soap in the spray will help the water/herbicide mixture stick to the leaves. It reduces the surface tension of water, which allows it to spred evenly over the leaves and not form droplets. 2,4-D and dicamba are similar, they are both growth regulator herbicides that attack broadleaf plants. Dicamba does give you a little more killing power, but it is much more expensive then 2,4-D as well. 2,4-D will also last a time period in soil, and soybeans should not be planted for at least 1 week after spraying 2,4-D.
If you are looking for a cheap way to kill all vegetation, mix roundup and 2-4,D together, you will like the results.