"I am going to attend" vs "I am attending"

Solution 1:

English can use both the present progressive construction, as in your first sentence, and going to, as in your second sentence, to express the future. Both describe a future event, arrangement or intention. However, going to suggests a little more strongly than the present progressive that the event is fixed and cannot be changed. The choice between the two depends on the context, and the impression that the speaker wishes to convey.

EDIT: This view is based on the relevant entry in ‘An A-Z of English Grammar and Usage’ by Leech and others. Carter and McCarthy in their ‘Cambridge Grammar of the English Language take a rather different view:

Be going to usually indicates that . . . the event will take place soon, but that all the necessary arrangements have not yet been made.

The present progressive usually indicates that . . . arrangements are probably in place or have been made.

You will see that they are careful to hedge their claims with usually. In practice, the choice between the two forms depends on the context, the impression that the speaker wishes to convey, the speaker’s purpose and the relationship between the participants in the conversation. For example, a speaker, asked to do someone else’s work tomorrow, might reply, ‘I’m sorry, I’m attending a meeting tomorrow’ and not ‘I’m sorry, I’m going to attend a meeting tomorrow.’ The difference is slight, but can be used to fine tune a speaker’s ideational and interpersonal meaning, depending on circumstances.

Solution 2:

The first "I'm attending a meeting tomorrow" gives the listener/reader almost 100% certainty that you will attend. The second one "I am going to attend a meeting tomorrow" gives the reader/listener the idea that you have the intention. You plan on it, but something could come up.

Solution 3:

1. I am attending a meeting tomorrow is giving the edge of present tense. It is a more finite decision where the speaker is detaching the event from his own will a little bit more as if it is all happening anyway.

2. I am going to attend a meeting tomorrow apart from conveying the same message can be used to express a more recent decision or resolution. Suppose you are making a choice of what to do tomorrow. If you have just made a decision, you are going to say: "I am going to attend a meeting tomorrow". You would not normally say: "I am attending a meeting tomorrow." in this situation.

The difference is very small in general, but sometimes important.