Cool deal. A build I've found phenominal, though I'm a first time cav player. Use a lance, for the love of god. I'm just using 2h for style purposes, and no shield because I'm mentally ill, but:
Str: 18
Agi: 24
6 PS
8 Riding
3 points left over for SHIELD***, Athletics**, or Ironflesh**.
I chose athletics, to move around a little better once I'm dethroned, and with 24 agi, you'll move pretty quick. Smart money on shield if you're a lancer, or use your sword with a shield with 1h wpf.
Smoothrich is right about weaponmaster, it doesn't do much for you. You're really just relying on timing and velocity. The difference between 6 and 8 riding is awesome, but as long as you can ride your horse, you'll put in work. 21/15 or 24/15 would work wonders. Even try 15/24 or 15/27 if you want.