You'll get sport mode anyway, but in case you're curious: It slightly changes the throttle response (relatively subtle, not nearly as much as the difference between eco and sport in a Volt), claims to change the steering sensitivity (I couldn't really tell the difference in casual experiments), makes the regen level you select with the paddle shifters stick instead of resetting when you use the accelerator (this is the most useful feature), reduces the energy conservation on the climate control, and makes the car way quicker to kick in the ICE for extra power.
In Eco mode the ICE only comes on for extra power if you push the gas pedal past the sort of "breakover" point near the floor. I don't like it, but as long as you're not stepping on it it's easy enough to avoid. In sport mode it won't use it for gentle acceleration, but the ICE starts up somewhere around halfway on the power meter/tachometer simulation gauge, so if you get even a little aggressive with the pedal you'll end up with the engine running.
Personally, that last bit was tremendously disappointing to me, and I never use sport as a result. Even "normal" will make the car use the ICE before you've hit the breakover point on the pedal (or it might relate to how quickly you push it; haven't really used either that much, since if I wanted an ICE running I'd have bought a regular hybrid).
