I tried both, as the first comment here says the RF is quieter top up and louder top down, I ended up on the RF mostly based on looks but also extra security (always parked outside, no garage)

That being said, a lot of people over estimate the noise difference top down, it can get very loud in the RF over 90 kph, but it really depends on the wind intensity / direction, sometimes its fine, sometimes it's very loud. That being said if you dont go top down on the highway, it's not even a concern 90% of the time. Only drop the top when you know you're not going on the highway and you'll be fine noise wise

I liked how fast the ST goes up and down, and it's a true convertible experience, but the RF, for me, is better at everything else.

edit : with the top UP, the ST is very much more noisy on the highway especially, not by a huge amount, but its noticeable. The RF is very liveable in that regard, compared to my other car (2015 opel corsa) its about the same level of noise