Nadal plays tennis of one and the same pattern. Murray plays various tennis, in which aggression is an option. Nadal genetically unable to attack, but some people think otherwise impressed by overpowered and overspinned moonballs that this player performs. On scale of 1-10, where 1 is full defence and 10 is full attack, Murray is able to play 2-9, Nadal 1-5. IMO.
well, my take on that is that murray definitely has more shots than nadal but he prefers to play defensively most of the time because he probably feels more comfortable that way. Nadal on the other hand is more stable in his shot selection because his arsenal isn't quite as big but he does try to be more agressive most of the time.
That said, tennis is about match-ups and with some players Nadal is a lot more defensive than with others. With Federer Nadal tends to rely a lot more on his topspin defensive shots than with other players. On the other hand, with Berdych for example, he tends to be more aggresive probably because he doesn't want an agressive flat-hitter to take the initiative and make him run around the court to win a point.