I can't say anything about using the canon one (being a nikon user and all)  and I haven't use the tamron either so I can't be that much help really!  But I've got the sigma 105mm F2.8 and its quite good, however it would be much better with some form of stabilisation, there's quite a few times I can't keep the shutter speed around 1/100 so I get some blurry pics and IS would make quite a difference.  There's also a few times when I wish I had something around 60mm but that just depends what I'm trying to photograph.

And to add more detail, the length gives you quite a bit of versatility, not only is it a good macro lens, but it's good for gigs as it has a bit of range for close-up-ish shots, and gives you good bokeh for non-macro but close shots of things, IE, portraits etc.

This was taken using Helicon Focus to stack together about 20 photos taken with the 100mm to get the whole image in focus and pretty sharp...


Lucanus cervus 1 by Chris_Moody, on Flickr

Went none IS yes. I have considered the IS version, but tbh the only times I've wanted IS on the lens is hand holding for macro work, and generally that has only been when I'm trying to get various critters, but as they tend to be moving around anyway IS wouldn't help. For everything else it's just easier to use the tripod. As most reviews say, the Non-IS/Non L version has IQ pretty much as good as its more expensive alternatives too.