macro lens choice

Field biologist to be shooting bugs and frogs and tadpoles; mostly in the field, some in the a studio setting. All on a Canon XS for now.

Canon EF 100mm f/2.8L IS $950

Canon EF 100mm f/2.8 USM $550

Sigma 105mm f/2.8 EX DG OS Macro Lens for Canon EOS Cameras $800

What do I want? In particular, is the OS on the sigma and the IS on the Canon L worth $250 and $350 extra? Could go for used copies on the more expensive two, if the verdict is that they are easily superior.

Thanks, all.