About frame material and ride quality....for anything other than racing conditions where you seek to eke out the last 1% of performance, tune your ride quality with tire selection and not frame material choice. The tires you run and the PSI you run them at will after ride quality far more than the type of material used to construct the frame. You can also add a Thudbuster (seatpost suspension) to help absorb bumps.
Frame material is more relevant in considering the longevity of the frame with the type of riding that you do. If you jump curbs, trees, logs (none of which you should be doing on a folder), then steel is probably a better choice for you. Under hard riding conditions, aluminium frames seem to last about 10 years. Cracks to alu. cannot be easily repaired whereas steel can simply be welded back.
The benefits of alu. is the lighter weight. But the frame tends to be thick to compensate for the relatively weaker strength of aluminium when sized identically with steel frames. So that becomes a matter for aesthetics.