Electronic gets send to FBI for them to do a check on you. It is a lot faster, since they are send electronically and they are already scanned. All FBI has to do is run them through the system and send the results back to USCIS.
INK on the other hand, has to be mailed to FBI. They it has to be scanned and ran through the system.
Also, electronic FP machine lets the operator know if they scanned the finger improperly, so they can rescan it right away. If that happens with ink, you will have to make an extra trip to be fingerprinted again.