Intentions in Present Continuous and Be Going to

While reading about the present continuous for future, I came across this idea in Advanced Grammar in Use:

We often use either the present continuous or be going to with a similar meaning to talk about planned future events. The present continuous indicates that we have a firm intention or have made a definite decision to do something, although this may not already be arranged:
○ Are you seeing the doctor again next week? (or Are you going to see ...?"
○ I'm not asking Tom to the party. (or I'm not going to ask...)

In the sentence I put in bold only the present continuous is mentioned. But as far as I know, we can use be going to for intentions. For example:

1. I am going to have a drink after work.

2. Jo has won the lottery. He says he's going to buy a Porsche.

3. We're not going to paint our bedroom tomorrow.

Does it mean that in present continuous the intention will be stronger, more firm and in be going the intention won't be that firm?

1. Bill and I are going to have a drink after work with the kitchen staff

at least reasonably strongly hints at (though this is context-dependent, and by no means demands) the unbleached sense of 'go' ... going to a pub, say. Also, this variant perhaps lends itself better to marking by emphasis (for instance, stressing are to imply 'whether you approve of this or not'). 'Going to have' will sound less weak here than 'having'. But these caveats apart, this phraseology often conveys what is no more than an unmarked, everyday statement in conversational (casual) register ... a statement showing confident intent (but hardly a bold assertion in the face of opposition).

2. Bill and I are having a drink after work with the kitchen staff

exemplifies what is usually an unmarked, everyday statement in conversational (casual) register ... again, a statement showing confident intent, not parrhesia .

So the two sentences are perhaps most often virtually identical in meaning and thrust.

...........................

Context matters. With

3. I'm not going to ask Jo to the party[!]                                         and

4. I'm not asking Jo to the party[!]

It is quite likely that the statement is marked for a sense of frustration, censure. I'd say that this makes (4) the more likely choice, as it is punchier and conveys the intended accompanying emotions more easily. (3) (minus the exclamation mark) sounds slightly more formal and measured, and is better where there is a plain statement of fact (perhaps Jo needs to rest / get on with her coursework) or the conveying of a well-thought-out decision.