Azure Blob storage: DownloadToByteArray VS DownloadToStream

Instead of streaming the blob through your server, you could download it directly from the blob storage. My answer is built on top of Steve's response here: Downloading Azure Blob files in MVC3. For downloading a blob directly from the storage, you would utilize Shared Access Signature (SAS). Recently Azure Storage has introduced an enhancement, which allows you to specify Content-Disposition header in SAS. See this modified code.

    public static string GetDownloadLink(string fileName)
    {
        CloudBlobContainer container = GetBlobContainer();
        CloudBlockBlob blockBlob = container.GetBlockBlobReference(fileName);
        //Create an ad-hoc Shared Access Policy with read permissions which will expire in 12 hours
        SharedAccessBlobPolicy policy = new SharedAccessBlobPolicy()
        {
            Permissions = SharedAccessBlobPermissions.Read,
            SharedAccessExpiryTime = DateTime.UtcNow.AddHours(12),
        };
        //Set content-disposition header for force download
        SharedAccessBlobHeaders headers = new SharedAccessBlobHeaders()
        {
            ContentDisposition = string.Format("attachment;filename=\"{0}\"", fileName),
        };
        var sasToken = blockBlob.GetSharedAccessSignature(policy, headers);
        return blockBlob.Uri.AbsoluteUri + sasToken;
    }

    public ActionResult Download(string fileName)
    {
        var sasUrl = GetDownloadLink(fileName);
        //Redirect to SAS URL ... file will now be downloaded directly from blob storage.
        Redirect(sasUrl);
    }

The benefit of Stream is that you can deal with bits piece-by-piece as they are downloaded rather than building up a big byte[] and then operating on the full thing. Your use of Stream isn't really getting the benefits since you are writing to a file and then reading that full file into memory. A good use of the stream API would be to pipe the download stream directly to the request's response stream as shown in the answer here Downloading Azure Blob files in MVC3

If you are planning to use the DownloadToBytesArray (async or not), you will have to fetch blob attributes first to get an initial size of byte array.

And if you will be using DownloadToStream you will not have to do that. That's one saved HTTP call to the blob storage and if I am not mistaken, FetchAttributes() is executed as HTTP HEAD request and that will count as a normal transaction (it will cost you some money in other words).