I went through a similar lens selection process recently. At the time, I had a 70-200mm f/4 IS and I wanted a macro lens. I liked the idea of the Sigma 150mm OS because it would give me a faster tele option to complement my zoom. In the end, I decided against this multi-tasking route and instead replaced my 70-200mm f/4 IS with the f/2.8 version and got the Canon 100mm f/2.8 to use solely as a macro.
I made this choice because I didn't like the idea of lugging around the Sigma (1,150kg) along with my f/4 zoom, just to give me a faster option. As it is, I can leave the Canon macro at home and cover my fast tele needs with just one lens. Getting the 70-200mm 2.8 IS also allowed me to get the cheaper non-IS macro since I invariably shoot macro with a tripod.
All that said, the Sigma 150mm OS is an excellent lens by all accounts so if it fits in with your needs, go for it. I suspect the new Sigma 180mm OS will be way more expensive.