您好！
首先，说说共同点吧！那就是：
动词不定式和定语从句作定语都要后置，即放在修饰词之后！
其次，说说不同点。
① 动词不定式作定语往往表将要发生的动作。e.g.
I have a meeting to attend tomorrow.
定语从句表示的时态更宽广！ e.g.
I have a meeting wich is to be held tomorrow.
The leaders are talking about the meeting that was held yeasterday.
The workers are discussing the meeting which is being held now.
② 引导定语从句是需要分清指人、指物、指时间、地点还是原因。而动词不定式不受此限制！ e.g.
This is the book that I want to buy.
She is the beauty whom I'm after.
I'll never forget the days when I woked in the factory.
This is the factory where my daddy used to work.
Could you tell me the reason why you were late again?