こととて・ものだから: the difference
こととてN1
Meaning
Due to, Since, On account of, Because
Past papers
Example
あの事件は２０年前に起こったこととて、世間からは忘れられているだろう。
That incident happened 20 years ago, so the public has probably forgotten it.
ものだからN2
Meaning
Because, The reason is
Past papers
Example
郊外に家を買ったものだから、通勤に２時間もかかる。
Because I bought a house in the suburbs, the commute to work takes two hours.
How to tell them apart
「〜こととて」: very formal, for letters and apologies; asks for understanding because of ignorance, youth or inexperience.
「〜ものだから」: a spoken excuse, personal in tone; not for formal business apologies.
Set phrases: 「知らぬこととて」, 「急なこととて」, 「子供のこととて」 (since he/she is only a child).