Both are excellent UUs that play off the leader(s) well. Keshiks have the extra advantage of the GER for extra experience right out of the gate and Genghis offfers faster GG production during the critical time of the Keshiks hey day. First strike and immunity to terrain is pretty sweet. On the other hand Numidian get a free promotion for a charismatic leader. As many of my horse archers get flanking I and II anyway, Hannibal can start Numidian with Flanking I, II, combat 1 which is pretty damned good in my book. Since they can go up the promotion line fast from Charismatic they can get formation fairly quickly which stands up to spears nicely. Of the two I would go with Keshiks but it's pretty close.