Having owned the FiST for 2.5 years and the RS now for about 2 months, I can say I have absolutely no regrets about trading the FiST in for the RS. I loved the FiST and it was a great little car, but the RS is on a whole other level. If you can afford it without stretching your wallet too much, then I don't think you will regret it. You might want to double check on your insurance though, I was quoted at an extra ~$20/month even when I told them it was an RS and then found out when I gave them the VIN it was much more than that (nothing crazy though, maybe closer to an extra ~40/month). The reason being is they quote you at the base model, so they basically just quoted you at what a regular Focus would be.
The ride is not harsher, that was one of my concerns but the RS (in normal and sport mode) takes bumps more smoothly than the ST did. It's not worlds better in terms of ride quality, but it is slightly better. I also have to disagree with the comments in here that the FoRS is only better when you are pushing it and the FiST is more fun at lower speeds and around corners.
In regular driving just getting from point A to point B the RS is unquestionably better, it has nicer interior, better technology (going from MFT to Sync 3), a smoother ride and it's a little more spacious. It still feels like a small car which I love and even when I'm not getting on it, it still makes me feel great and puts a smile on my face. The seats and steering wheel along with the gauge layout and positioning of the shifter just makes the car feel much more race-oriented than the ST.
You absolutely do not need to be pushing the car to 9/10ths to have fun in it. It is pretty easy to kick the back end around on that car and I've done it around a bend at ~30mph just putting my foot 4/5 into the throttle. Yes, you definitely have to be more careful driving the RS than you do the FiST, but at the same time it's not a Ferrari where you can never push it to the limits on the street. Someone made a comment about the FiST being better/more fun around corners and that is not true. Where the RS really shines is handling... it holds a 1.04 lateral G where the ST had a .92 I believe (from Car and Driver). I only kicked the back end out of my ST once on the street in the 2.5 years I owned it and I have done it twice now in two months in the RS. The headgasket issue is a serious one and I read through quite a lot of pages on the RS forum before I just made a decision to get a 2018.
Ultimately only you can answer this question and I am just sharing my experience. I loved the ST and wish I could have had both cars but I have no regrets with the RS, it is a wonderful machine.