'or is cavalry better all the time?'
Cavalry was only better in certain situations, and in certain parts of Europe. For example in the west, the emphasis was always more on footsoldiers due to the relatively high amount of population (compared to eastern Europe) and the fact that areas of habitation were very close together.
Cavalry was not 'better all the time', it's better in certain situations. But since EU simulates battles in a more abstract form (personally I want them to be more detailed, perhaps strategic or tactical for EU II) it's difficult to understand.
Cavalry is obviously faster, and players who have an advantage in cavalry in battle get certain attack bonuses. However cavalry is a complete waste when it comes to sieging operations, it simply bleeds away.
Personally I use cavalry throughout the entire course of the game, depending on who I'm playing I usually have one 3 foot soldiers compared to one horse, i.e. 3:1 ratio, but that can change depending on circumstances.
As far as I remember, the more you progress the more 'bonuses' you receive with regards to the infantry when gaining military tech's..e.g. better organization, better firing rate and so on.
Sapura