Basically, there are three main ways to judge the ideal route to climb:
Fastest, most picturesque, and least busy.
The Marangu Route is the fastest and can, in theory, be done in 5 days. Fastest has the side benefit of being cheaper too.
Fastest may sound good to the super-fit, but a person's ability to deal with the low oxygen air found at altitude is not related to fitness. The best way to improve anyone's ability to deal with the effects of "thin" air is to walk slow and spend more days on the mountain.
The Marangu Route is a busy route with high traffic.
Note: Because of its low summit success rate, we do not recommend the 5-day Marangu Route climb.