1. 意思不同。
come round：苏醒；顺道拜访；改主意，转向；复原。
drop in：v. 顺便走访；顺便访问； 偶然拜访。
2.用法不同。
come round直接接宾语，表示拐弯；否则后面要接to+地点或是to do表示拜访哪里或是去干什么。

drop in如果拜访的意思，后面需要接介词at或是on构成固定词组。否则表示什么下降。
drop in on sb/sth：拜访某人；drop in at sp.：拜访/参观某地。
All right, I thought you'd come round.
好啦，我本来就认为你会改变主意的。
Why don't you come round for a meal this Sunday?
这个星期天你何不到我家来吃餐饭？
Some people will always take longer to come round to the news, but with these patience and consistency will usually win out.
很多人在接受新的信息的时候，会比较慢，但是对付这些人，耐心和持久力通常十有效。
More than half of visitors drop in on the page because they've heard about it from friends.
超过一半的访问者都顺便走访了该网页，因为他们从朋友那里听说过。
Greek debt continues to drop in value as some officials now admit that earlier rescue loans were not enough.
希腊债券价值持续下跌，部分官员现在承认，这是因为早期救援贷款不足。
Please drop in at my office when you happen to be free.
如果你碰巧有空的话，请到我的办公室来。
Next time you are in Guangzhou,please drop in on me.
下次来广州请一定来看我。