Our advice is always choose the highest number of days limited only by your budget and schedule.
The main reason for not summiting Mt. Kilimanjaro is AMS (Altitude Mountain Sickness). The best way to prevent AMS is to spend more time acclimatising. Which means to spend as much time as possible on the mountain and above 2500 meters.
Every route on Kilimanjaro has variants depending on the number of days. All routes offer a variant where you can spend an extra day on the mountain to help with acclimatisation.
So, the solution is simple - spend as many days on the mountain as possible. This solution unfortunately comes with higher park fees as a cost. Even then, the increase in park fees is a very small percentage of the overall cost of your trip.
In general, the increase in summiting success rate and overall less stress is definately worth the additional cost and time spent on Kilimanjaro.