If you or your listener have just said that Datong is very hot, it's suitable to say ‘我听说西安比大同更热’. (In speech, this sentence is stressed on 更.) Otherwise, say ‘我听说西安比大同还热’. Note that this sentence is stressed on 大同. And it implies that Datong is already well-known as a extremely hot place.

在你给的这个句子里， “我听说西安比大同还热’ vs ‘我听说西安比大同更热” 还和更是一样的， 但是， 在有些句子， 还和更就有细微的差别， 比如， 你比我早来1分钟， 这个时候， 可以说你比我还早来一分钟 但是这个句子不能用更， *你比我更早来一分钟 是不对的。 也有些句子是只能用更， 不能用还的， 比如 “ 没有比老师更高尚的职业了” *“没有比老师还高尚的职业了” 是错误的。

我觉得你说的没错，就是有点感觉不一样。 我更喜欢说（这里就不用“还喜欢说”了），"我听说西安比大同还热." ，"我听说跟大同比，西安更热" 因为"还"平常用起来， 语气上带埋怨，比如，“他还不回来” ，“他怎么还不回来”。 更，就没有这种意思，一般说好的事情比较多，比如，“她更美”，但不说“她还美”

1 "大同好热啊!""我听说西安比大同更热." —— You have just said that Datong is very hot. So use 更, and stress 更 in speech. 2 "我听说西安比大同还热." "啊?! 真的?" —— You haven't said that Datong is hot. But if Datong is well-known as an extremely hot city, you can say "我听说西安比大同还热." without having to mention it first. In this case, you must stress 大同 in speech.

there is no difference. ur teacher is correct.