Which of these two statements seems to be more appropriate?
- I am attending a meeting tomorrow.
- I am going to attend a meeting tomorrow.
I am quite not sure which one to use.
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:
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.
Both forms mean exactly the same thing. I think that a speaker may subconsciously feel that adding unnecessary verbiage like "going to" gives the listener time to prepare mentally to process the words most likely to follow.
The phrase "go ahead and..." is likely the most often used unnecessary phrase in English. For example, I've heard the following on a DIY show: "Go ahead and measure the board. Now go ahead and cut it. Then go ahead and sand it." Leave out that phrase and no information is lost.
Another possible subconscious purpose of using such "empty" phrases could be that speaking concisely may sound curt, or brusque, and thus unfriendly, while adding empty phrases makes the speech sound more relaxed and friendly.
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.