if you have to go only with one of these 2 I'll go with i5-7500 .
the 2 CPUs are rather close in performance to each other and for you stated you're going to building it for a pure gaming ...
well in my op I go with intel when it's just gaming .
you might use a computer for streaming games, video editing, image editing.... The 1500X is likely to do as well or better than the i5-7500.
for the ram 2400mhz is good considering the CPUs you're going with ,not a big noticeable difference between 2400 and 3000 on this scale so go with what you prefer.
so it's your choice