Canon 100mm f/2.8 Macro USM or Sigma 150mm f/2.8 EX DG HSM

I am considering the sale of my 50mm f/2.5 Compact Macro and an older 35-70mm f/3.5-4.5 to partiallly fund the purchase of a new macro lens that will give me full 1:1 magnification. The 50mm Compact Macro is only 1:2.

The Sigma is slightly more expensive than the Canon. Most of that is due to the 150mm vs 100mm, I would imagine. I do not know anything about the build quality of either lens.

Does anyone have any experience with either of these two lenses?

I have the canon 100mm macro and even though i havent used it much, the times i have it has produced nice clear and sharp pictures. One of my co-workers who has something to say about all the pictures i and others take was impressed with the sharpness of the lens. Hope this helps!

this is one i gave some thought to ( not got one yet ) but the sigma is what i decided on its as sharp as the canon and the extra lenth means you wont frighten the bugs by been to close .
Rob.

I can't speak for the Sigma but the canon works great and doubles up nicely as a portrait lens.

Well I have the 100mm Canon. This is a very well used lens. It makes a good telephoto lens also for general use.

At life size there is about 140mm of working distance from the front element. This is enough for most bugs, even dragonflies. The only problem bugs are butterflies. When my wife lets me I want to get some tubes to try on my 200mm for them (the EF 180mm macro is expensive and heavy).

The 150mm should give more working distance which is normally valuable but too much working distance can be a problem in undergrowth. Not shure what the weight differnce is but the less weight the better for handheld bug chasing.

Another point of you have a 20d but no macro flash you can get reasonable exposures with the internal flash with the 100mm (no hood), I don't know about the XT. The 150mm may be too long for this.

Consider how each focal length would fit into your system.

The Sigma 150 includes the lens hood and tripod mount in the purchase price. The lens hood is a necessity because it protects the front element. The tripod mount is a great convenience because it lets you swing from landscape to portrait orientation without having to re-position the camera or deal with the off-balance weight of the camera hanging out to the side of the tripod in portrait mode.

If you add the cost of hood and tripod mount the Canon 100 macro is not cheaper than the Sigma 150.

Thanks to all for the replies. I have some time to make the final decision as I have to either come up with all the money or go ahead and sell a couple of my lenses to offset the purchase.