Locked vs unlocked phone

I want to purchase an unlocked S9+ through Samsung. I will use it with AT&T pay as you go. The phone is a little cheaper unlocked. Does anyone know what the difference is between the two? Is one 4 GB of Ram and the locked is 6 GB of Ram? Which I guess isn't that big of a deal since my current phone is only 2 GB of Ram. Why the price difference? Fewer features? Is it really worth going unlocked?