RS is good but the potential is only really unlocked once you void the warranty and go stage 2. The gains are insane with stage 2 and a tune (see below). However, if you look at the RS vs STi they have very similar numbers without upgrades... not sure it's worth it to pay a lot more for a RS. Probably driving both is the best way to see which you like better though. Subaru seems to be keeping the same motor for the 2018 STi as well.
(1:30 is the dyno chart)
http://www.caranddriver.com/comparisons/2016-ford-focus-rs-vs-subaru-wrx-sti-vw-golf-r-comparison-test