When you start a new game, one of the early decisions that you will need to make is whether to build cavalry or horsemen. The limited number of battle slots means that you probably should not build both unless your civilization is very small.
At first glance, cavalry seem like the obvious choice, since their offense and defense are double that of horsemen. With offense/defense of only one, horsemen tend to die in job lots when in battle, especially if they are in heroic stance. A hit that will 10 cavalry will kill 20 horsemen.
However, cavalry also cost twice as much as horsemen -- so if you lose 10 cavalry or 20 horsemen, this represents 1000 hammers in both cases.
The math turns in the favour of horsemen quite substantially if you add in any +1 offense/defense wonders or civics. A +1 to a horsemen brings it from 1 to 2, doubling its strength. A cavalry unit is only improved from 2-3, a 50% increase. If you have +2, your horsemen are tripled, while cavalry are only doubled.
As well, horsemen are pretty much the only unit that can benefit from snow (at least after the early game until fighters show up and I have never seen fighters). This can be used to your advantage, but it also means that they are less useful in other weather situations. Cavalry can at least stand up against spearmen.
The biggest issue is that it is really hard to sit there and watch your horsemen die in large numbers, which leads people to tend away from putting them in heroic stance. Unless they are there as a placeholder, why use units that you are unwilling to fight with?
From a market point of view, cavalry are usually more expensive than horsemen. You can often find very good deals on horsemen, while cavalry usually cost more than hammers. Even if I have chosen to build cavalry, I usually end up with a bunch of horsemen because I couldn't resist a good deal.
What do others think? Are horsemen better than cavalry?
EDIT: or just get enough of one of the other to warm a slot until you can get knights?