Mountain bike or road bike for steep roads?

Which is less difficult to ride up a steep tarmac road: mountain bike or road (drop handle) bike? By 'steep' I mean 1:3 to 1:5.

Mountain bikes tend to have lower (granny) gears, but road bikes are much lighter. Or am I wrong?

Detailed explanations will be awarded points!

Cheers.
8 answers 8