There is no difference in meaning here between will receive and will be receiving, and nothing in the context which requires one other than the other.
What is involved here is that (at least in the US—it may be otherwise in ‘Anglo’ Englishes) ordinary conversation has taken a dislike to will VERB: it is felt to be rather stiff and bureaucratic, perhaps because it is the form ordinarily used in official communications. The version with the progressive is ever-so-slightly softer and friendlier—not fully conversational (which would be something like you'll be getting), but a little more casual.
The sentence, "I would leave tomorrow", is grammatical, but is lacking some sense (subjunctive mood)
The Subjunctive Mood
A verb is in the subjunctive mood when it expresses a condition which is doubtful or not factual. It is most often found in a clause beginning with the word if. It is also found in clauses following a verb that expresses a doubt, a wish, regret, request, demand, or proposal.
Source: http://englishplus.com/grammar/00000031.htm
"If I would leave tomorrow..." (correct)
ALSO:
(1) I was going to leave tommorow. <- WRONG (tomorrow indicates a future time reference, you cannot use it considering the context you provided)
"I am going to leave tomorrow." (correct, the day before leaving)
"I left [yesterday]." (arrival)
(2) I was leaving tomorrow. <- WRONG (same as above)
"I will be leaving tomorrow." (correct, the day before leaving)
"I left [yesterday]." (arrival)
I will not discuss the use of past progressive here, but I'll provide a link http://www.englishpage.com/verbpage/pastcontinuous.html
*I left tomorrow. <- WRONG (same reason)
You cannot say, "I ate tomorrow" in the same way that you cannot say "I will eat yesterday".
per definition:
tomorrow
[tuh-mawr-oh, -mor-oh]
noun
1.
the day following today:
Tomorrow is supposed to be sunny.
2.
a future period or time:
the stars of tomorrow.
Since 'tomorrow' is a future time reference, it is wrong to use it in the above quoted sentences. The point here is you cannot simply use a future time reference in a sentence referring to a past action. Also, it is ungrammatical to say "I would leave tomorrow" (in the context the OP provided). 'Will' is a modal used to indicate future tense, hence the main verb's tense should be changed. As above explained, you may use it if you are indicating a subjunctive mood.
Remember that leave/leaves (present); left (PAST); and will leave(future).
Best Answer
It's more formal and polite, and also somewhat less obligating. It's more formal because it doesn't feel as halting as "When will you leave", and in English, letting things flow over an extra couple syllables can help a lot. It's also more polite, because "When will you leave" sounds like you're interested in them being gone, not in the last chance to see them before they go. And it's less obligating, because leaving is something that can take a while, you can go around and say goodbye to everyone before you step out, maybe you'll get caught up in a conversation or two, and you might stay a while longer even though you should be going... So, you can claim to be leaving up to 30 minutes before you actually plan to step out the door.
That said, it does sound a little too formal for casual events. A more everyday way of saying it is "When are you planning to leave?" That keeps the politeness and light obligation while still sounding friendly.