I have the original and the sport. I love both, but I prefer the original. The original has seams (I want to say they're called bullets) where DD's bottom sits. She sits better in it than the sport.
Also, the sport's back is about an inch taller or so. When DD was shorter, the sport rubbed a raw spot on the back of her neck if she was in it for very long.
I live in Texas where it's hot! DD and I are both hot when I wear her and I honestly can't tell that either of us is cooler in the sport.
None of these are deal breakers, just minor differences. You'll love whichever you choose!