Ok, first thing is first are you trying to make a living off it? If so you going to need some money to invest and quite abit of it if you want to make a decent amount. Now more profitable from my point of view "WAS" D3 but the AH is getting removed next patch so there will be no point in botting in D3 unless you want more toons/items for yourself.
With WoW you can make some money but you will also need to invest some money then it takes time to build on that and get a return, if you dont get banned in the process. At the moment I bot for fun and just to help my guild out and friends I did bot for money years ago when it was a lot better for pricing today its just pocket cash.
I am not saying you cant make a living off it but plan on how you are going to start out and how much you need to spend while playing it safe
Razer