I own both set of rules. The Horse and Musket tends to have bigger units per player with 9 or 10 figures in some cases, as opposed to a suggested five per player in the core rules.
The horse and musket is focused on the time period and weapons, and whilst black poweder weapons are mentioned in the basic core rules, they are looked at in a bit more detail in the horse and musket set.
As I want to play F&IW games with the rules, I went ahead and purchased the additional rule set.
If you want to play games with various settings, with not so much focus on black powder weapons, I think the core rule book is a better buy for you.
Wiley Game Designs do sell the rules on special from time to time, so you can try out one set and look at maybe getting the other one at a later date.