I was supposed to be there 31 Aug - 6 Sep. But, with the storm threat, I don't know if it's really worth it. If there is a storm, will it affect the conditions for long time, or go away after a day? How long can a storm last? Will it ruin beach time for all 7 days? What are the probabilities?
Here are the current NHC predictions for Tropical Storm Erika. Note that the shaded areas are not the affected areas; they are the potential areas where the center of the storm might end up. A swath around the actual storm track will also be affected, which (depending on where the storm goes and how large it is at that time) may or may not include Miami for an indeterminate amount of time. Currently, the forecast is for this storm to be fairly slow-moving (it looks like it'll take 48 hours to go the length of the Florida peninsula), meaning that the actual stormy weather might last for a while.
If you're planning to travel by air, I would keep an eye on the advisories issued by your airline. Here, for example, is Delta's. Note that as of this date (8/28, 3:30 EDT), they are allowing passengers traveling to the Bahamas, Hispaniola, Puerto Rico, and several other Caribbean destinations to rebook or cancel their reservations without fee. (If they are cancelled, then you get a voucher for future travel rather than a refund, alas.) I would watch the website for your airline; if Miami is added to the list, then you definitely don't want to be there.
None of this answers your question for what the beach conditions will be like, of course, but hopefully someone with more Florida storm experience will be able to address that.
As an aside: I only lived in Florida for one year, but one thing I remember my friends telling me is that a storm that passes by well offshore can actually briefly improve the weather. They said it had something to do with the low pressure offshore "sucking up all the humidity" or something like that. I don't know whether this is actually a thing, or whether it was just a confirmation bias on their part. It's interesting as local lore, at least.