Hi Lee,
Depth of field is determined by image magnification and aperture. So if you are achieving a certain level of magnification (say 1:1), and are using the same aperture,
any lens/focal length will give you the identical depth of field. So the 180 and 100 do not differ in the depth of field, given the same magnification and aperture. They will differ somewhat in 'perspective' but this is different from depth of field.
The 180 will enable you to increase your working distance, which might make lighting a little simpler. In nature work, that longer working distance is very helpful if you're photographing bugs, butterflies, etc., where you can't get too close without spooking your subject. In your case, it's just a matter of convenience and lighting.
At high magnification, depth of field is limited, requiring small apertures. This is true regardless of which lens you use to achieve that magnification. I've used my 100 macro down to f/22 or even f/32 to get the depth of field I want. I'm not sure what that friend meant by it being "useless" beyond f/16. Theoretically, you might run into some sharpness problems due to "diffraction" at very small apertures, but this is probably not all that significant under normal circumstances.
With small apertures like f/16, f/22, f/32, lighting does become an issue, since you need a lot of it. Natural light shooting will necessitate long shutter speeds and tripod mounting. Tripod use would still be pretty much mandatory even if you're using flash or studio lights.
As for Canon vs Sigma, that's the perennial question. Personally, I prefer to spend the extra $$ and go for the Canon. Better resale value down the road could come in handy should that need arise. But that said, there are many people quite pleased with their Sigma's performance, and the extra $$ in their pockets.
Here's some macro shots taken with the 100 (and some extension):
http://www.dlcphotography.net/Macro/MacroFrameSet.htm
Good luck!
Don
http://www.dlcphotography.net
I am really in a quandry about which lens to get. My desire is
leaning toward either the Canon 180mm 3.5L Macro lens or maybe even
save some $$$ if the performance is identical and go with the Sigma
180mm Macro 3.5EX HSM. My second thought is to go for the 100mm 2.8
Canon or the 105mm 2.8 Sigma. As usual the Sigma's are 1/2 the
price of the Canon's , but I really am not so much concerned about
the money as the performance. My application is going to mainly be
small jewelry (rings and pendants (not chains). I shoot the catalog
for CDROM for the company I work for, and I want the best results
possible. Someone that owns the 100mm 2.8 Canon told me that the
depth of field is VERY limited indeed and that stopping down beyond
F16 is USELESS. I rather like having a bit more depth of field to
get the entire ring or pendant structure IN FOCUS. I also would
have thought that a 180mm would allow me to get further back from
the item as that would be an advantage for me. I would like to have
a lens that I could use outside of this purpose as well. So would I
get better depth of field control with the 180mm or the 100, and if
so should I just save my $$$ and get the Sigma or is the
performance of the Canon actually superior? Thanks in advance for
any advice from folks out there who have been here with this
decision before and found the answer.