RX is the same except the shaft is a core shaft instead of solid. Stihl doesn't recommend using any attachments with the RX. Save yourself the trouble and get the FS85. All of mine have been extremely reliable and lasted many many hours.
Let me tell you about my new trimmer....
I just got the new Honda HHT25S. This is a nice machine! Very quiet, no dirty, oily smoke, a solid machine, hopefully very dependable! Always takes awhile to get used to new equipment, but I'm liking this thing a lot so far.
Honda has been making 4-cycle trimmers for awhile but I did not like the UMK models. I tried one of those last year and it was no comparison to the Robin BH2500's that I have been using. But Honda has listened to customer feedback and made the new ones lighter and more powerful. I also did not like the head, which has been replaced with the best heads I have used - the same ones on the Robins.
The HHT25S is 25cc and lists for I think around $340. My local Honda dealer has it for $299. Another shop in the city sells it for $309. It weighs about 13lbs. The HHT31 is heavier and more powerful.
Someday all trimmers will be 4-cycle (and then maybe electric?).