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.
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.
It's easy with steel to go outside of the realm of easy repairability. In the last days of steel before aluminum became affordable and even before carbon fiber did, it was common to for fancier bikes to have tubing that changed thickness (interior of course) once or twice, as stiffness at the ends of the tubs tends to matter more, and it's easier to weld thicker pieces of metal.
And then you had Serotta, who before they switched to titanium, made steel frames that were not just double-butted, but they also spirally ground the butting to maintain most of the torsional stiffness but lose a couple more ounces of weight.
Yeah, but try getting carbon repaired after a crash. It's not impossible, but it's also not as easy to work with as a material. That said a lot of new high end carbon bikes will come with crash replacement on the frame.
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.
https://youtu.be/1CTjg1TFHDc