Based on this other answer, a new question arose. When is runway slope most important, at take-off or at landing? Or is it equally important in both flight phases?
I would argue that it is most important on takeoff. The reason is that most planes require more distance for takeoff than for landing (partially due to higher weight). Also, it is generally easier to abort a takeoff than a landing (when on the ground at least).
Both landing and takeoff distance calculations are performed (which should take slope into account) to ensure that there is sufficient distance for the aircraft. However, in addition to aborting a takeoff being easier, more can go wrong in a landing. Windshear or improper approach settings can cause an airplane to touch down later or faster, which risks an overrun. During takeoff, the plane has until V1 (takeoff decision speed) to abort if any issues arise.
However, this certainly depends on conditions. Aircraft condition, wind and runway surface condition can increase the distance needed for landing (or aborting a takeoff). If reverse thrust is not available, there is a tailwind, and the runway is wet, that could change the situation.
I would rather land downhill than takeoff uphill because taking off uphill you have just one thing to get you off the ground: thrust (ok, maybe a little flaps helps in some airplanes). On landing, I have a lot of things I can control: approach speed (slow), approach angle (steep), approach power (none), flaps (full), flaring before the threshold, aerodynamic braking with the elevator, pulling the flaps, full braking. some of it is admittedly pretty gutsy, but you can stop even a slippery airplane quite quickly.
I am going to introduce another perspective. We are talking about a road of 3km length with a potential difference of 1% (30m!!!!) between the highest and lowest place.
Runaways are always thought to have enough visibility for the pilot when trying to land of take-off. Limitations of the slope in airports are not related to performance of the airplane, instead is a safety and visibility constrain.
So, either I am taking off or I am landing I prefer in both to be running uphill where my airplane at the fastest speeds, so I will be able to see any object really far from where I am. No matter the performance I lost.
Notice, that we need to consider also the visual range of the pilot (which is several meters above the road) which is not 3km for small objects, but, I prefer the safest condition: Uphill.
The only time you would worry about runway slope is insufficient runway. So let's consider a runway overrun:
On takeoff, there's a period where you should no longer abort takeoff, as there is likely insufficient room for stopping. That happens as the plane accelerates pass V1. At that point the aircraft is quite some way down the runway travelling fast. It's heavy as well. If a late rejected take off is initiated, you'll likely hit the fence at a very high speed.
On landing, you plan to stop the plane anyway. If things go awry, you can always step on the brakes. Landing distances are never computed at max braking so you got some room. If you overrun, you'll hit at a much slower speed, since the plane is slowing down in the first place.
Therefore I'd argue it is more important on takeoff.