my most successful build ever was the 24/18 i had for a while. that was before the heavy lance damage nerf, overhead turn nerf and before most people were high level or fully loomed, so take that into account.
since then i always had 21 agi 7 riding 7 athl, and i am quite positive that it ups the gaming experience. 7 athl helps you choose your fights a lot better, 7 riding seems to have a nice edge concidering turn rate and acceleration, but is not that necessary. the arab is a great horse, i didnt use one for very long cause i am a courser boy, but now it somewhat replaced my unicorn cause its just more fun on slower maps...
(the feathered horse you talked about in other thread has 8 riding, so forget that : )
anyways: 21/21 will give you more freedom, i prefer it, but 24/18 is a great build...