I like to tell that I am saving money to buy Mac Book . So two sentence come to my mind , which one is more suitable to use ..
- 僕はMACBOOKを買うためにお金を貯金しています。
- 僕はMACBOOKを買うためにお金を節約しています。
I would suggest 「お金を貯{た}めている。」or a more polite conjugation if circumstances require.
節約 would mean you are being 'economical' with your usage of money, rather than emphasizing the saving of money.
貯金している might be alright, but I believe it has a stronger connotation of the physical accumulation of money, rather than the more abstract sense of just 'saving money'. Also, as @naruto さん mentions, it is tautological.
For No1. You are putting money into somewhere you can rely on such as Postoffice or something for buying MACBOOK. 貯金 is normally talking about actual amount. Putting money into Banking is normally called 預金, I might say 貯金 to my friends though.
For No2. You are saving money just stopping smoking Tabacco to reduce spending money than previous month in order to buy MACBOOK. This is talking about the efficiency rather than the actual amount see the definiton of 節約