IMO, the ERGO sport feels less substantial than the ERGO. Comfort-wise I can't tell any difference, but I can see why it is slightly cheaper by the look and feel of it. It is somehow smaller. Also, the seat for baby is not darted. This gives the seat of the Sport a less smooth and fitted look than the Sport, although as far as I can tell, there isn't much different in comfort for baby.
I've got a chart listing all the differences here: http://www.carrymeaway.com/pages/ERGOComparison.html
Hope this helps!
Laurel
Owner, Carry Me Away
Mama to Jack, 5 and Maddy, 3