I haven't used these lenses (I use Nikon) but one thing to consider is
that with 60mm macro (I have a 60mm macro) you need to get quite close
to the subject -- I mean, a couple inches from the front of the lens.
If you're doing insects, that may be a consideration since that may not
be practical.  The longer focal length gives you a few more inches of
room to work.

I have the EF 100mm macro, and I really love it. You can see some of the
things I've done with it here:

http://www.pbase.com/mschiff

I agree about the comment that the 100 lets you be a bit farther away than a
60. Even with the 100, sometimes it's difficult to get close enough to a
very small moving subject.

-- Martin

No, but I like the Sigma 105 macro lens.

I don't know much about the 60mm, but if it is a 1:1 macro, it will give
the same magnification as the 100mm, but you will need to be closer to
the subject to get it.  The situations in which you are shooting are
very important, as is the perspective you wish to achieve.  A shorter
focal length allows you to get closer to the subject for the same
magnification, which may be good or bad.  If you're shooting a hornet,
the 100mm will give you more distance to work with.  If you're shooting
the top of a high flower, the 100mm may have to be too high to focus.

The backgrounds will be different, as well.  The 60mm brings you closer
to the subject, but not as much closer, percentage-wise, to objects in
the background (assuming equal magnification), so they will be
relatively smaller than if you had used the longer focal-length lens.

The DOF and the needed shutter speed for sharpness will be the same, for
the same level of magnification.  This is very important to understand,
because common wisdom is that the shorter focal length lens requires a
slower shutter speed, and has more DOF, but that is only true at
*infinity*, and becomes more untrue the closer you get to the subject.
With the same magnification of a subject, shutter speed is the same for
all focal lengths (unless you have IS), and with the same f-stop, the
same magnification yields the same DOF, so *NEVER* decide to use a macro
based on focal length alone, to address DOF or shutter speed!  Consider
focal length to address things like positioning of the camera/lens, and
relative magnifications of things at different distances from the
camera/lens.

You may to check with the DP Review, 350D group, too. There are several
very good macro shooters there.
---
Slack - trying to save for a Sigma 180

Hi.

I have the Sigma 105mm macro and a friend has the Canon 100mm macro.
The Canon is much faster to focus (using an EOS 30 body).
The Sigma has a switch and a ring. Both have to be set to Auto or Manual
focus and the ring is _very_ easy to move unintentionally to the Manual
setting. I believe that Sigma's instruction leaflet warns against the ring
being in Manual setting and the switch (and therefore the lens) being in
Auto setting.

Personally, I'd go for the Canon macro if you can afford it.

Regards,  Ian.