@KD2022,

The official site has a compare tool that would prove useful in most situations, however not so much in this specific case.(Comparison Between RE700 and RE715X)

In general, these offer the same functionality and performance, it is the mainly the difference between having internal and external antennae. Technically speaking, you will be able to get better coverage with external antennas as they can be directed and are not confined to the inside of the RE.