They are both an evil that prevents life. The difference, however, in non-abortifacient contraception is that the egg has not been fertilized and therefore no human person exists. Once conception actually occurs there exists an entirely unique being, separate form the mother or the father; a human life. The fact that it is in the first stages of development is the only difference between that fetus and you.
As for your myth that we have an overburdened planet, if you really wanted to do something, why don’t you just sacrifice your own life rather than taking the life of a completely defenseless, innocent human being who never agreed to that arrangement?
The reality is that you have to do neither. We do not have an overburdened planet. We have governments who keep their populations in poverty as a means of control. We can feed the entire planet just from our country alone. And we attempt to feed many, but the grain and other goods sit on docks and rots, or is taken to feed the those in whatever military régime that happens to be in control. “Want to eat? Then join us or starve.”
Your statement that “Both have the consequence that there are fewer mouths to feed” is absolutely disgusting. Families that loose a child also have one less mouth to feed and they mourn that fact for the rest of their lives. How they long to feed that mouth. And every one of these little ones that you would kill in order to have one less mouth to feed are no different.