The difference between these two forms is explained here: http://www.talktomeinkorean.com/lessons/level-3-lesson-6

It is difficult to explain the difference. But let me try it. --------------------------------------- #점심에는 밥을 먹을 거예요. #Announcement of a decided thing: The menu for lunch is set, we'll have rice. / We have decided to have rice for lunch./ It is decided that we will have rice for lunch. / We are told to have rice for lunch. --------------------------------------- ##점심에는 밥을 먹을 게요. ##I will do it according to the demand/hope of the listener.The listener wants/hopes the speaker to have rice for lunch. Other examples: 오늘은 늦어서 미안해요. 내일은 일찍 올께요.(Sorry for being late today. I will come earlier tomorrow.) 오늘은 열쇠를 가져오지 않아서 참 미안해요. 내일은 꼭 열쇠를 가지고 올께요.(Sorry for forgetting the key today. I will bring the key tomorrow definitely.)

Maybe a better example: 내일은 잊지 않을 거예요. It is assured/secured that I will not forget it. I can't for get it (even if I want to forget it). In this case, you are telling the listener that it is secured, it doesn't depend on my memory. For example I have taken some special measures to prevent it from happening. (For example, I have told my mother to remind me of it tomorrow morning. So, even if I forget it, my mother will tell me to take it tomorrow) 내일은 잊지 않을 게요. I will try not to forget it tomorrow. / I hope I will not forget it tomorrow. / I have a strong will not to forget it. In this case, nothing is secured(it still depends on my memory). I will try not to forget it, but no special measure is taken to prevent it from happening.