I have the sport & I love it. We tried them all on at the p&b expo and the sport was so much lighter & the straps were more comfortable on the shoulders. I started using it when bub was 1 week old, the insert is great. I love my ergo, it's easy to use and better for bub than other carriers.
The sport was our choice because we have a summer bub & in winter she will be dressed warmly anyway.