とあって・にあって: the difference
とあってN1
Meaning
Since...
Past papers
Tested once in past papers
Example
いつもより講演の参加者が多いとあって、発表者はいつも以上にソワソワしています。
Since many people attended the lecture, it is no wonder that the speakers are uneasy.
にあってN1
Meaning
At, In the
Past papers
Tested 3 times in past papers
Example
好況にあっても破産を申請している企業がある。
Even during a boom, there are companies applying for bankruptcy.
How to tell them apart
「とあって」: a rare scene caused by special circumstances, such as a shop packed on the first day of a sale.
「にあって」: being in a hard or special period or situation, as in 「戦時下にあって、文化を守り抜いた」 (Even in wartime, they protected the culture).
「ので」 and 「から」: ordinary reasons.