I'm presuming these bikes are built off the same base frame with just different components, so the biggest factor will be the largest rear gear cogs. More teeth in rear = easier climbing. The number should be listed on each bike's product page, and will usually be seen as the range (ex. 11-50T, where you're looking at the second number). This is also assuming the front ring has the same number of teeth between the two.
Another thing I'd consider looking at when comparing is if there's a difference of the front forks using coils (springs) or air for the suspension. If it's in your budget, go for an air one. Even if you're not doing jumps or drops, the air will take care of any rocky chatter better, since coils have a tendency to rebound quickly and buck you back after it soaks the initial hit.
An actual REI employee can probably jump in here and offer more specifics.
one or more gifts or other benefits from the co-op.