verb forms - "I should go" vs. "I should be going"

I should / "had better" be going.`

I should go.

What is the actual difference between these two? I hear the first one more often.

Answer

As far as the verbs themselves are concerned, the difference is between an action that ought to be completed at some point and an action which you ought to be in the process of doing. In other contexts this is a little more obvious.

I should go visit her before she leaves.

I should be going to visit her more often.

In the first example visiting her is simply an action which you need to complete at some point (or, in this case, before the deadline of her leaving). The second example refers to a continuous activity — the process of paying her multiple visits — and the fact that you should be doing it more.

However, I'm inferring from your question that you mean specifically the use when you're trying to say that it's time for you to leave. In that case, the second has an implied time (now), which makes the strict definition pretty interchangeable:

I should be [in the process of] going.

I should go [now].

The difference in this usage is mostly in the tone.

"I should be going" makes it sound like there are circumstances beyond your control which are forcing you to leave. Perhaps it's getting late, you're getting tired, or you have prior arrangements you need to take care of. It's polite and you should certainly not have to worry about offending whoever you're speaking to because of this wording.

"I should go" could be interpreted as being rude. It isn't, strictly speaking, but it's the way I would word the phrase if I was leaving because I had an issue with the place I was in. Perhaps I was fighting with the person I'm speaking to, my ex just showed up and I don't want to deal with him, or I otherwise feel people here would be happier if I left.

I can use "I should go" in a more neutral situation as well, but then it usually has reasoning attached to it. "I'm starting to fall asleep here, I should [go/head out]," or "I should go, I don't want to be late for the movie."