clearance. With this rim, it looks to have the same rim depth for all of the offsets offered which leads me to believe you will still have the same desired amount of lip as the +30 offset. My logic with the +38 is you will be able to fine tune your front offset with spacers which would give you more flexibility for running a 265/35/18 tire up front without rubbing issues.
I have rpf1's up front in 18x9 +38 with a 265/35 tire. I needed to use a 10mm spacer to clear my Z1 13" bbk due to my specific rim design. I still have no rubbing issues and im lowered on Tein Flex coilovers.