I am sure lots of folks on here are more knowledgeable than me, but I think torque might be more important than horsepower. Some engines that came out a couple of years after mine have a much greater level of torque relative to the horsepower and from what I understand keeping the RPMs at a higher level under load will help power up the hills.
I did a quick search, and this looks like a good article that explains what I am trying to say...
http://www.dieselhub.com/tech/horsepower-vs-torque.htmlI believe you can think of torque as being similar to leverage - how much force does it take to move a certain weight? The more torque/leverage, the less force it takes to move something of a certain weight.