Which Macro for the Spring?

[Glenn72]

I am thinking about adding a macro to my lens collection to photograph bugs and butterflies with my Canon 450d in the upcoming Spring.
In looking around I have decided upon these two lenses

1. Canon 100mm f2.8 Macro (non IS)
2. Sigma 150mm f2.8 Macro

The IS models of each are above my budget and I will be using a tripod anyways.
I am thinking the Sigma will give me more reach for bugs and also provide better bokeh as it is a longer lens. Any thoughts on which would be better for bugs and butterflies?
I already own extension tubes but not sure how well they work with macro for bugs as with my 50mm prime I have to be almost touching the subject to get into focus. Does extension tubes work this way with Macro lenses?