So many formats, so little time

I've been racking my brain over the last few hours trying to determine the best format(s) to save data into for a fairly large project I've got coming up. Basically, I need to export data sitting in a caching tier I built from scratch in C# into some sort of structure that will populate an Excel spreadsheet that constitutes the data source for a separate application that reads tabular data and generates dynamic TV graphics. The centralized caching tier is used to ensure synchronicity across multiple, diverse platforms that have clients calling the data repeatedly. (See my MSDN article on this very topic for more.)

Since this is an internal utility, I've got an admin control panel on my machine running Office XP, with the separate machine on the network share onto which the Excel data will sit using Excel 97. I'm torn between just programming against the Excel API and writing the data file to the mapped drive on my LAN; or writing an XML file locally to disk (much easier programmatically) and having the spreadsheet read the remote data source across the network share; or using the Office Web Toolkit and having the spreadsheet routinely make asynchronous SOAP calls to generate fresh data via a web service. (I've got to figure out a polling mechanism for this latter approach, though.)

Each has its ups and downs and levels of complexity, and none is too far out of reach. I'm just going nuts trying to figure out the most-right fit. Me and my perfectionist syndrome.