I faced the same question last year; currently I have both the 100 Macro and 580EX II. I've never regretted buying the the 100/2.8 Macro first. It is great for both macro and low light situations such as weddings. This photo was taken with the 100/2.8 using available light.


http://naturesbeststudents.blogspot.com/2009/02/need-for-speed.html