Depending on the insect you may want to go longer than 100mm - I have the Canon, and some insects, especially butterflies, are kinda skittish. You can do it, but it takes patience.
For insects, working distance (distance from front of lens to subject at maximum magnification) is important. According to
this article:
Tamron SP 90mm f/2.8 Di Macro Lens 3.9" (99mm)
Canon EF 100mm f/2.8 USM Macro Lens 6.0" (150mm)
Sigma 105mm f/2.8 EX DG Macro Lens 4.8" (122mm)
Sigma 150mm f/2.8 EX DG HSM Macro Lens 7.6" (194mm)
The article's author thinks that the Canon is worth the higher price (even including price for the lens hood).
I've noticed a trend here - when people ask for opinions between a few lenses, most people would recommend the most expensive automatically without any consideration as to suitability. However, in this case, those who recommend the Canon said why they'd choose it over the others, and the price difference is not prohibitive.
BTW what are the prices you've found for the 3 lenses in Singapore?