Jim,
Writing an HTML file (even with formatting) is going to beat the pants off of sending (just) data to Excel.
Try my FAQ vs. sending the data to Excel via automation on a few thousand records and you'll see what I mean. And note the FAQ is demonsrating how to apply conditional formatting too.
The one performance hit is when Excel is opening and interpreting the HTML. In the files I create this way I've automated the open-and-save-as-Excel before .visible=.t.
And of course, Excel 2000 + is required to read load the resultant HTML files...
Brian