Don't listen to the idiot above me.
Your horse's speed and manouverability are key, they get you the kills.
The horses you have, are all for different situations. Overall, the horse with the most balance between manouverability and speed is best. While armoured horse may have more charge and survivability, that wont do you any good because of the recent developments. Everyone has a pike these days, plus cavalry is mroe numerous, one on one manouverability is key.
Hm I stayed with the heavy courser for a looong time, and it's fine, but now I realized I AM a speedfreak, so I got riding 8 at lvl 30 before I last retired, it wasn't enough for me, so now I finally go for the champion courser, I think it will take one arrow less, but I hope they will have more difficulties to hit me, too.
So I would say, if u don't plan to retire that often for a horse, stay with the heavy, it's fine. If you are really into riding and need the speed and stuff, do >75% of your kills from horseback, go for the champion, but I think spirited (between that) is a waste of time (at the moment I am there :-( )