Still deciding...UWA or Macro

I still cannot decide wether or not to get the Sigma 10-20mm UWA or Canon 100mm f/2.8 macro. It is not about comparing the two, but rather if I really need UWA or really need macro.

I will be purchasing the 70-200 f4L, though, and I do have a WA (18mm kit lens) and an 85mm f/1.8 lens.

Is there distortion on my 1.6x 350D with Sigma @ 10mm? Is there a big difference between 10mm and 18mm? It'd be more like 16mm and 28.8mm, right? Cause of the 1.6x crop body...So would I be looking for a photo that has examples of 16mm and ~29mm? Or 10mm and 18mm comparisons?

Would macro or UWA contribute to my collection most? I do not use UWA very much, aside from school projects in my photo class

Thanks a lot for the votes, and the help...
Matthew

Oh, and my budget is $500. And I'm already set on the 70-200 F4L

So... do you want to shoot macro or landscapes?

If you like macro, get the 100mm, no question. It's absolutely awesome for macro. If you want to do UWA, get a wide angle. They aren't really comparable...

I think you'll need to decide what your priorities are first, and then we can help you with lens selection.

^Ditto...

I went UWA first. No you can't stand back and focus, but you can get a pretty close focus of a stationary but still no macro.