Just some general thoughts, so please understand I am not prescribing one over the other. The ride quality of a bike is largely dependent on frame geometry and manufacturing process. With a good layup, and geometry, carbon can be more stiff then steel. This is actually the promise of carbon, to be stiffer then steel while being lighter. However a good quality aluminum bike can also be lighter, and more stiff, then a carbon bike with a low quality layup. Steel tubing also comes in a variety of different qualities.
https://gravelcycling.wordpress.com/2016/07/07/steel-the-dif....
The obvious benefit to steel for me is the reliability and repairability. Ride quality can also be different depending on surface, steel is better at dampening vibrations then aluminum on gravel surfaces. Although carbon can obviously be quite nice, and you are seeing it used in tough ultra distance rides like the GDMBR. That said your wheel choice, and seatpost, will have a bigger impact on ride quality then frame material as far as vibration. Generally speaking the weight of your bike is going to be most noticable on climbs. How noticable will also depend on your gear range.
This video about titanium bike myths discusses some of the points I outlined above in further detail.