首先,词义范围大小不同。since较之后者较大,除了相同做连词外,还有介词、副词词性。since做介词时,与现在完成时态或过去完成时态连用。如:I haven't eaten since breakfast.我吃了早饭后到现在还什么都没吃呢。
其次,同为连词时,since和because有重叠的意义,即因为,由于。since还有既然之意。
同做“由于”讲时,二者区别在所处的位置上。一般来讲,since较多置于句首。
如,Since we've no money we can't buy a new car.
I did this because he told me to.
习惯上,because在后面,英语习惯先果后因。
参考资料:oxford advanced learner's english-chinese dictionary