In terms of price, you will not guess in what moment of the week it will be the most favourable, so the only thing that you will regulate by buying in one day or every day is fees that you spend. If you buy just paying for transaction fees, it is better to buy 1400$ straightaway as you need to pay a fixed fee no matter how big your transaction is. Nevertheless, if you use a centralized exchange where you have to pay, for example, 1% from the whole sum, it doesn't matter for you what way to choose.