You need to add a "neither."

I use Python/Sage/NumPy, and when I want to send stuff to non-techie Windows users, OpenOffice Calc.

I use both. For the most part I stay in Numbers but for other many things Excel on the Mac is still better then numbers. For example, Quickbooks will export reports directly into Excel but not into Numbers. Also the 'Text to Columns' feature works better in Excel then in Numbers. In general I think Excel is a more powerful program then Numbers. However on the Windows side Excel beats both mac applications hands down.

For my modest needs, OpenOffice Calc does the trick.

This poll will reveal more about what you use spreadsheets for than anything else. Excel is often used as a Mini-OS, running programs and interfacing with databases, the web, etc.

I've never seen Numbers used that way.

Hacking in Excel is much like hacking in any other language. I love the power it gives me and the fast development of analysis spreadsheets.

The graphs are ugly by default though, but 2008 made strides to fix that.

Google Docs Spreadsheet here.

NumPy

Gnumeric

R

Excel