use what the recipe calls for - some you can use margarine in place of margarine - some you can use only shortening - some call for just butter - some even call for oil - and some can be a combination of shortening and butter

if substituting butter for shortening - your cookies will turn out a bit different - of course butter is healthier for you because margarine and shortening are made with hydrogenated fat -

shortening will give you a cookie with a crisp edge and a chewy middle

butter makes your cookies crisper throughout and because of the moisture content in butter, the cookies will spread more during baking

if you prefer butter over shortening and you want a more cake-like cookie, add an additional egg or egg yolk to give you that texture

Very well said Mama Mangia, hope this explains the difference of using a shortening.

thank you!