Choosing between a colon and semicolon when both are appropriate

Question

Let's say you want to write one of the following:

I am very happy; I just won $100.

I am very happy: I just won $100.

It is my understanding that both of the above sentences are grammatically correct. How, then, would you decide which to use? Is there a subtle difference between them?

Answer 1

Depends on what you want to say.

As stated in the comment, colon says

and here's why, or because

so your sentence would be

I'm very happy because I won $100.

That seems to be what you want to say, because they are correlated.

But a semicolon separates independent clauses, and can be written with a period. So you are saying,

I'm very happy. I won $100.

So it depends on you, but 99 out on 100 cases, go with my 1st one.