Its always good to buy bitcoins in installments and in small amounts as if you do so then you always have an opportunity to buy in future if price drops again so it will help in accumulating more coins.
Its good to buy large amounts, the only thing is that you need a lot of money to do this and the fact is also that with some services its not possible.
If you want to do this you can as best go find a near dealer on localbitcoins than can help you out.
I think buying in large amounts would not be possible for everyone as it will also require good capital to buy them so follow your budget and buy that amount that you can afford.