Leave/go

The versions with the progressive are far more likely.

There is no difference in meaning?

As 'now' in this sentence suggests 'at this moment, then it is difficult to think of contexts in which the present simple could be natural.

Then what about these?

Let's go/leave now.
Let's be going/be leaving now.

TakaThen what about these?Let's go/leave now.Let's be going/be leaving now.

The non-progressive bare infinitive is probably more likely.

In this utterance "now" is probably marginally less immediate. - ''let's" tells us that the going/leaving' is not happening at the moment of speaking.

Related question.

Do you think these are exactly the same in meaning?

The train is leaving this station at 6 p.m.
The train leaves this station at 6 p.m.
The train will leave this station at 6 p.m.

TakaRelated question. Do you think these are exactly the same in meaning?The train is leaving this station at 6 p.m.The train leaves this station at 6 p.m.The train will leave this station at 6 p.m.

No, they do not have exactly the same meaning, although they refer to exactly the same situation.

Then could you kindly explain what the difference is?

Generally speaking, when talking about future situations:

the present simple is used for scheduled situations,
the present progressive is used for situations for which some arrangement has been made;
BE going to is used for situations for which there is present evidence;
will is used to express the speaker's certainty and/or volition.

The choice of which

"I leave now." sounds like something a king or emperor would say.

"I go now." sounds like something a 19th century general would say.

"I'm leaving now." is what someone would say today. A man would tend to say this more than a woman.

"I'm going now." is also what someone would say today. A woman would tend to say this more than a man.