It's interesting the fact that from 18th to 30th, there's only one player outside the OHL (Oscar Dansk) and nearly the half of the list is from OHL (14/30). At ISS, 12/30 are from OHL. If we put in front the numbers, we can see that they are pretty similar:
Button & ISS
OHL: 14/30 - 12/30
Europe: 6/30 (5/6 SWE + 1/6 FIN) - 5/30 (4/5 SWE + 1/5 RUS)
WHL: 5/30 - 5/30
QMJHL: 3/30 - 2/30 (Bussières)
USHL: 2/30 - 6/30 (Matheson, Skjei, Schmaltz and Kerdiles)
It seems that ISS take more into consideration the USHL than Button, who fixes more at the top leagues. At this moment, both list ranks players from the same leagues. What are the chances of players from other leagues, like Cristoval Nieves or Jujhar Khaira? We can see players from another countries like Czech Republic (Tomas Hertl), Slovakia, Switzerland, Germany or Denmark?