Steel versus MDF/Aluminium Y axis plate?
I have not used MDF for building a printer before; but, I have used it for other projects.  It has the advantage of being very flat (initially); but, it has a LOT of issues with moisture.  It is basically just a compressed slurry or water-based glue and sawdust.  If you expose it to humidity or water it will swell like a sponge.  I would not consider it for anything that requires a dimensional stability.  For that, Aluminum is your better bet.

Regarding the material properties of AL vs MDF, here is a good comparison:


Note that while MDF has about 1/4 the density of AL, it has a MUCH lower Elastic Modulus (1/17 of AL) For the same thickness, it MDF is MUCH easier to bend than AL.  Also note the strength to weight ratio of AL is also better.

Even at twice the thickness, my calculations indicate that, for the same load, 6mm MDF would deflect about twice as far as 3mm AL. Also when AL exists its elastic region it becomes plastic (bends) where MDF breaks.

Another aspect to consider is flammability.  There a lot of heat sources around an FDM printer and if you are planning on a heated bed, there is one right there under the bed.  Where MDF is hard to ignite, it is flammable and does not respond to heat well.  On the other hand, AL can handle temperatures over 1000 degC and is a great thermal conductor for a bed heater.

I would definitely choose AL over MDF for you printer bed. 

Another option to consider it is using a bare PCB (like FR4).  The material is really strong (it is fiberglass), is relatively inexpensive, and is fire resistant (hence the"FR" in the name).  Some commercial printers use FR4 for their print bed.  One disadvantage is that is can sometimes develop a curl and there is really no way to get it flat again.