You can use both form here. Both '으니까' and '어서' is used for showing the reason of the following action.
But sometimes they cannot be used in certain sentences.
For example, '어서' cannot be used before imperative sentences while you can use '으니까' before them.
e.g. 시간이 없어서 서두르세요. (X)
시간이 없으니까 서두르세요. (O)
Here are some idiomatic expressions using them.
e.g. 만나니까 반가워요. (X)
만나서 반가워요. (O)
도와주시니까 감사합니다. (X)
도와주셔서 감사합니다. (O)
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