I don't speak from specific knowledge of these materials in reservoirs,
but rather from a general woodworking background.  Each has their
strengths, and I have routinely used both in a wide range of projects.

First, please be sure to distinguish MDF (medium density fiberboard)
from particleboard, another manufactured sheet product.  Both are
products created from waste wood and glue, but the differences are night
and day.  In MDF the wood fibers are dissociated and then pressed back
into sheets with a resin to bond them, producing a very dimensionally
stable product with a very smooth surface that machines well, takes a
very nice paint finish and makes an excellent substrate for veneer.

On the downside, MDF is not particularly strong, will readily deform
under load, and will swell if exposed to moisture.  It is also
exceptionally heavy: a 4' x 8' sheet of 3/4" material can weigh 110
pounds -- no fun to move around the shop.  It also produces very fine,
pernicious sawdust, requiring good dust collection and personal
protective equipment.  Vast amounts of cheap modern furniture is made
from it.

Particleboard, on the other hand is sawdust compressed with glue,
lacking the intertwined fibers of MDF.  It is very weak, it has a rough
surface that cannot be made really smooth, and it falls apart with any
amount of moisture.  Even vaster amounts of _very_ cheap furniture is
made from it, particularly with laminate surfaces.  I have never
discovered a role for which particleboard is the preferred product,
with the possible exception of protecting a floor from traffic.

Plywood products cover a wide range of quality and properties.
For organ work I think the only product worth considering is plywood
made with all plies of hardwood (typically birch, alder, or maple) and
sold as "Baltic birch" or "ApplePly" (a US-made brand name).  The real
stuff is made with cross-banded plies that are 1.5 mm thick using
exterior grade glue and is completely free of voids.  18 mm material
(~3/4 inch) will have 13 plies, compared to the typical 5-7 plies of
softwood plywood.  It is dimensionally stable, very strong, relatively
lightweight, but can warp some with differential moisture.  (Finishing
both sides the same typically eliminates this problem.)

Beware that the local Borgs ("Big Orange Retail Giants, resistance is
futile!") are selling a lookalike product from China which has
microscopically thin face veneers, many internal voids, and a tendency
to warp like a potato chip.  Most of the real stuff comes from eastern
Europe and is sold as 5-foot-square sheets, with the US "ApplePly" sold
in 4' x 8' sheets.

While I think both products would work for reservoirs, my strong
preference would be to use the hardwood plywood, based on its lighter
weight, resistance to sagging, relative ease of working with it, and
aesthetics.  The cost of the two products is close enough to not be
a material consideration.

All that said, there's still nothing wrong with the traditional
solution of solid wood boards glued together into a panel with some
nice newspaper or wallpaper applied to it using hot hide glue.  It will
be lighter and stronger than any sheet product, and the newspaper
provides interesting reading to the next guy who opens up the
reservoir!

Cheers,
Roger Wiegand
Wayland, Massachusetts, USA
http://www.carouselorgan.com/