Steel versus MDF/Aluminium Y axis plate? I have purchased a P3Steel v.4 kit that, rather unfortunately, comes with a steel Y plate.

I understand that there may be interia, as well as stepper motor wear, issues related to the weight of the steel plate. Therefore I would like to substitute the steel plate for another material. I had considered aluminium, as recommended by the RepRap wiki - P3Steel/Frame Versions/ Version  4.0 :

We recomended use aluminum beds for y axis.

However, it was suggested, by a vendor, that I use laser cut 6 mm thick MDF, principally for reasons of economics and availablity, over aluminium.
I have subsequently found a supplier of 3 mm thick aluminium 200 mm x 300 mm Y axis plates, so availability is no longer an issue, and the slightly higher cost is not really an issue for me. However, I was wondering whether there would be significantly less interia if using an MDF Y axis plate, than with an aluminium plate.
I assume that the masses/densities of aluminium and MDF are comparable, and a magnitude less than that of steel.
I have found the densities of aluminium and steel:

Aluminium 2.7x103 kg/m3 (167 lb/ft3)
Steel 7.82x103 kg/m3    (488 lb/ft3)

[Source: Densities of Solids - Engineering ToolBox]
The density of MDF is given as 700–720 kg/m3 (43.7-44.95 lb/ft3), which is a magnitude less than that of aluminium.
[Source: Medium-density fibreboard]
There is the issue that MDF can eventually warp, whereas aluminium supposedly does not, although the OP of Wanhao duplicator i3 print bed support warped shows that warpage is possible with aluminium plate - see Is the weather a problem for MDF frames? and Would a steel, instead of an aluminium, plate be reasonable?1.

TL;DR
Considering the benefits of weight/inertia, stiffness and lack of deformation/warping:

Is the difference in density between steel versus aluminium and/or MDF, make it worth switching from steel to aluminium or MDF?
Is the additional weight loss of MDF over aluminium worth the risk of deformation of the MDF due to humidity/temperature?


Notes
1 I found this question after having written up my question, so I admit that there is a risk of duplicity of the answers. However, I am concentrating more of the comparison of aluminium and MDF in this post, rather than just steel versus aluminium. Also, my question deals with the Y axis heatbed support, rather than the heatbed itself.
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.