Help with buying weapons in Final Fantasy 15

Hello everyone,

This seems like a pretty easy thing but I am having some trouble with it. In FF 15 when I buy weapons, how I do I tell if the weapon is actually stronger than the one I have currentally equipped. Same goes for shields and accessories.

In the other games it was way easier to tell as you scrolled through them, a little up or down arrow came beside the character so you could tell but I cant figure it out in this game. I keep buying weapons and they wont be as good as the ones I have equipped. Some lower my HP or magic and I am just so confused.

Thanks