The biggest difference between the Lemosho and Northern Circuit routes on Mount Kilimanjaro is the way they circumvent the peak. Both start in the west of the Mount Kilimanjaro National Park, at Lemosho Gate, and are identical for the first two to four days (depending on the variation).
On the Shira plateau, or latest at Lava Tower, they split: The Lemosho Route continues along the popular and most scenic southern slopes to join with the Machame Route, while the Northern Circuit branches off to avoid the busy southern circuit and circumvents the peak on the rarely visited northern slopes of Mt Kilimanjaro, near Kenya.
In terms of difficulty, we consider both routes roughly equal. At 72km (44mi) of total length, Lemosho is a little shorter than the Northern Circuit, which is the longest route on Kilimanjaro.
Depending on its variation (8 or 9 days, including or excluding an acclimatization detour to Lava Tower), the Northern Circuit has a total length of 80km (49mi) to 94km (58mi).
Therefore, the Northern Circuit takes one more day to complete: While Lemosho can be completed in 7 or 8 days, the Northern Circuit takes 8 or 9 days.
The average distance covered per day, however, is 9-10km for both routes. Therefore, even though the Northern Circuit is longer overall, we do not consider it to be more difficult.
In terms of acclimatization and summit success chance, we don’t see a major difference as both routes have a very high summit success chance (read the golden rules of altitude acclimatization).