Really depends on your playstyle. Arabian is a great horse but dies very fast.
This is my build, I die easily when hit but its a pretty fast build, also when dehorsed I can stay in the fight with 7 ATH and a backup weapon. If you want to use Lance only at the beginning and then switch to 1h/shield you could put all the points in 1h instead of polearms and maybe not use athletics but go for shield instead.
Strength: 18
Agility: 21
Hit points: 53
Skills to attributes: 8
Power Strike: 6
Athletics: 7
Riding: 7
Weapon Master: 3
Polearm: 133