Well first of the wide angles - the sigma and canon have been compared a lot and the result is that the sigma gives the same quality of photo for half the price - go for the Sigma lens (its one I am also aiming to get)
As for the macro if you are shooting insects then a key part is your working distance - the longer it is the further away you have to be to shoot at full sized macro whilst the smaller the closer you have to be - and of course the closer you are the more chance there is of spooking the insect. The factor that determines your working distance is your focal length - the longer the focal length the longer the working distace - so the canon 100mm has a shorter working distance than the sigma 150mm. That said 100mm is the bread and butter insect focal length - shorter is not impossible, but harder for certain whilst longer helps.
Now as to the lenses you list the canon 100mm is a very popular lens and works very well (infact all the macro prime lenses are very sharp and well made so you really cannot go wrong). The canon has a disadvantage in that it comes without a lens hood or a tripod collar - both of which are important. The hood is almost a certain need and has a lot of uses (besides keeping light controled its great for keeping things from accidentally scraping the front of the glass); The tripod collar is not as essential, but my experiences are that it does help with the overall stability of a lens setup on a tripod over using the conection on the base of a camera.
The sigma 150mm does ship with both of these attachments (and is the same price as a canon 100mm with both hood and collar ordered) and has a longer working distance than the canon. It also has teleconverter compatability (sigma teleconverters) which is important since teleconverter boosts your focal length either by 1.4times or 2times (double the focal length) - which of course means a longer working distance, but also affects you background blur effect, since the longer the focal length of a lens the more blur you get in background which is important for reaching clear backgrounds to subjects; further magnification of the macro is inreased when you use a teleconverter - which lets you get even closer shots of the bugs. However they do make 2 things harder - for starters the longer focal length will reduce your depth of field (the area of a shot in focus) and in addition it will also reduce your plane of focus - in the case of a 2 times this is very small indeed and can make handheld macro very hard.
A 1.4times can be used easily with no reduction in overall photo quality - whilst a 2times is trickier since your plane of focus and depth of field are so small - I have shot handheld with this combo and it is hard - I would rate it as a tripod macro tool more than a handheld simply because of its limited focus plane - though you can still have fun trying

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All the macro lenses you list are well suited to handheld macro work. Myself I went for the sigma 150mm as well as a set of teleconverters and its been a fantastic lens to work with
Further as for the lack of IS in the lenses its not essential at all. Landscape shots really should be done on tripod anyway (timer or remote release for added reduction in shake - also look up mirror lockup - a camera feature - which also helps reduce camera shake) so you don't have to worry about a longer shutter speed.
Macro work is nearly always done without auto focusing and without IS (infact only one lens had shake reduction and at the macro settings its not even adviced that its used - the nikon 105mm mirco (they call macro mirco do nikon people)). So the lack of IS is not limiting in any way. I would suggest a good flash (430 or idealy a 580M2) is a very important investment when doing macro work, though if you cannot afford one yet then by all means use your popup - but make sure to put a few folds of toiletpaper held over the flash (some elastic bands) to diffuse the light otherwise it will be harsh on your shots. A dedicated flash head will also need a diffusier - lumiquest softbox is a wonder - though again toilet paper can work very well as you wait for funds to build up
