It's kinda hard to compare these 2 games because they are completely different games, even though they are both shooters.
GoW is a third person shooter in which there is no real strategy. Your use of cover is different and you can heal teammates. Grenades are used differently and the objective of the game is different.
Rainbow 6 uses cover to scope out the enemy and use covering fire to advance your teammates. Use of different weapons, such as grenades, smoke, tear gas, etc, can be used to an advantage or disadvantage. You cannot heal teammates and there's more strategy to the game. Plus it's in in first person, which limits visibility.
Like I said it's unfair to compare the two side by side.
Can anyone else back me up on this?