Examples: 말 안 하는 게 낫겠다. I'll bet I'm better off not to talk about it. 말 안 하는 게 낫겠대. She said it would be better for her not to talk about it 말 안 하는 게 나을 거야. It'll be better for me(or you or him/her) not to talk about it. 지금쯤 자고 있겠다. S/he will be sleeping by now. 지금쯤 자고 있을 거야. S/he would be sleeping by now.

Both can be used to refer to one's intention or decision, but "-겠-" conveys a stronger sense than "-ㄹ 것이다". In these cases, "-겠-" only works for the first and second person except for quoting the third person's statement. \^o^/

In the case of verb '하다', we can speak 하겠다 or 할것이다. Both mean 'will do'. Two of them are not so different but -겠다 includes one's will more, I think. But they are similar so you can speak both.