Unless you really need VBA macros and you really need extremely high-level functions that Excel offers, go with iWork.
If you're only moderately interested in VBA, you can probably do all that with AppleScript. And although Numbers function list is smaller than Excel's you'd be surprised at what you could accomplish with a "lesser" spreadsheet.
And finally, I've seen few people rate Powerpoint higher than Keynote.
mt
Unless you really need VBA macros and you really need extremely high-level functions that Excel offers, go with iWork.