Hashem says that Koleiv will be rewarded. What about Yehoshua? Further what was this Ruach Acheres, a different spirit that the Torah speaks about?
The Sholol Rov brings from Rav Yehuda Kahana (brother of the Ketzos) the Gemoroh (Brochos 34b) that says, "Bimokom Shebaalei Teshuvah Omdim Afilu Tzaddikim Gemurim Ein Omdim Shom." Baalei Teshuvah ascends to high levels that Tzaddikim cannot reach.
Moshe Rabbeinu Davened for Yehoshua and therefore Yehoshua was a Tzaddik from the beginning of the mission until the end without wavering for a second. Koleiv felt pangs within him drawing him towards the mistakes of the rest of the Meraglim. He fought them off relentlessly and broke away to shed tears on his own behalf before the Avos HaKedoshim in Chevron. He struggled against his Yetzer and emerged victorious. He was a true Baal Teshuvah and for this earned a special place that Yehoshua could not reach.
The Possuk says that Koleiv had a "Ruach Acheres", a spirit that was pushing him to go with the Meraglim. "Vayimalei Acharai", but nevertheless he followed Hashem. Therefore, Hashem promised him a special Sechar (reward).
