Firstly, you haven't said which ROG Strix z390 motherboard you are refering to, so for this I'm assuming the Asus ROG Strix Z390-E.
The main differences between the Strix and the Maximus are:
. The Strix is an ATX sized motherboard, while the Maximus is a Micro ATX size.
. The Strix supports upto 64GB of RAM while the Maximus only supports 32GB of RAM.
. The Strix has more ports for SATA 6 Gb/s than the Maximus.
I can't recommend either to you without knowing the rest of your build and what you plan to use this machine for.