If you don't want to break the bank I would go for the Sigma 180mm macro. It's decent quality and gives you the reach for nervous insects. However, it depends on what you photograph. If you do not photograph nervous insects, then the 105mm would be fine and would be easier to use i.e. lighter and more compact. I'm not sure I would use a TC though.
This might be of interest:
http://www.nnplus.de/macro/Macro100E.html
I am a little confused though. Isn't the 105mm lens you mention a macro?
Leif