You will find conflicting opinions on both from various players. McGrath has described Tendulkar as the most mentally tough player he's played against, however he never felt like he'd take the game away from him to a huge degree, and he could generally control him to a certain extent. He found Lara mentally weak, but once Lara was in and playing well, he was harder to control.
Murali just flat out found Lara more difficult to bowl to and believes he was the better batsman against him.
Warne I think favours Sachin but correct me if I'm wrong.
The reason they are compared so often is because they are both so great and so different bowlers found one better than the other. They are so similar in greatness, but also so different in how they achieved that greatness.
I am again reminded of Top Cat's comparison of the two. Greatest metaphor ever. Something about Lara pissing on a canvas or something