There's some basic stats here (though intended primarily to compare the performance of different versions of PHPExcel) but they also show the differences in speed and memory usage between CSV, Excel5 and Excel2007.
The main difference is the completely different structures of the files, and the way in which they can be read/written. At the moment, we use SimpleXML for reading Excel2007 and XMLWriter for writing Excel2007 (version 2.0 will use XMLReader for reading Excel2007 to reduce the memory usage, but at a slight cost in speed), whereas the Excel5 reader and writer are working with an OLE2 BIFF binary object, so there's speed overheads there in converting packed data to PHP datatypes, but less memory overhead; and the OLE2 code more easily allows seeking in the data. CSV, of course, has the advantage of not supporting any formatting, so there's no overheads for styles.
The main difference is the completely different structures of the files, and the way in which they can be read/written. At the moment, we use SimpleXML for reading Excel2007 and XMLWriter for writing Excel2007 (version 2.0 will use XMLReader for reading Excel2007 to reduce the memory usage, but at a slight cost in speed), whereas the Excel5 reader and writer are working with an OLE2 BIFF binary object, so there's speed overheads there in converting packed data to PHP datatypes, but less memory overhead; and the OLE2 code more easily allows seeking in the data. CSV, of course, has the advantage of not supporting any formatting, so there's no overheads for styles.