I had a 2011 RS and I traded it in on the 2015 F3-S. I like them both very much. Here are my thoughts;
Styling......to me F3 hands down, looks more aggressive and muscular.
Power......this is a close one, the RS had more midrange punch, the F3 has more top end pull. Both are fun.
Sound.....RS hands down. Nothing beats the V-twin sound. I had a Two Brothers exhaust on it, sounded so good.
Handling....if you like to hang off the side, I still think you can corner faster on the RS. If not you'll be happier cornering on
the F3.
Suspension....front goes to the F3, the RS had better rear suspension geometry. If you are riding two-up, you'll definitely
need a better rear shock on the F3.
Comfort.....F3! I had to add handlebar risers to the RS, it was fairly comfortable after that and I did like the foot peg
position to stand once in a while. Overall the F3 is much better though and for passengers as well.
Storage....RS. The front trunk is way bigger.
Reliability....my RS was extremely reliable, never let me down. The F3 has also been reliable, no problems yet.
Maintenance....the F3 is definitely easier to maintain.