I think it may be difficult to directly compare laminate or coated hardshells with softshells, because the technology involved can be complicated.
So here's the simple answer: If you get a strech-woven softshell, it will almost certainly be more breathable than any hardshell, at the expense of complete waterproofness. At the same time, most softshells are quite a bit warmer than a simple hardshell, because they provide natural insulation, and thus are also heavier.
If you are climbing Kilimanjaro in the dry season, you probably don't need to worry about a downpour, so I would assume that's why they would recommend a softshell. There are some highly breathable hardshells, but they are usually specific to a sport or style, so choose wisely.
Things to focus on are:
- How waterproof do the pants need to be? If they need to sustain a downpour, go with a hardshell pant (they will not be stretchy, they will have the phrase "Waterproof/breathable," and they will probably have either a polyurethane coating or a laminated PU membrane). If they are to protect from the occasional sprinkle or snowstorm, go with the softshell. Waterproofness has no perfect standard of measurement (and companies that use Gore-Tex don't post their numbers), but for scale, a 15,000mm rating is pretty much 100% waterproof, with certain caveats. Here is an article describing "waterproofness": Waterproof Ratings Demystified.
- How breathable do the pants need to be? If you are performing lots of aerobic activity (like hiking steep hills at altitude), you will burn up without breathable pants, so get a softshell. However, if the pants are just there for emergency downpours, I'd go with a super-lightweight hardshell, because they will weigh less than the softshell, and you'll just take them off when the rain goes away. There is no perfect standard of breathability, but it is measured in grams/24hrs/meters-squared, and that is often abbreviated to g/m2. Pants at 10,000 g/m2 are pretty darn breathable.
Example of a highly breathable hardshell: GoLite Malpais Jacket
Example of a highly breathable softshell pant: Marmot Scree Pant
Read up on the trip reports at SummitPost.org
Check out REI's Rainwear Guide
and Backcountry.com Softshell Buyer's Guide