Materials

• Carbon fiber vs fiberglass:
  

Carbon fiber is often used to make RC structures. The dynamic soaring gliders are typically built exclusively from carbon fiber. 
On really light weight structures, light weight carbons are very, very, very expensive. Some can be $200 per yard while an equal volume of fiberglass is $5 a yard.
If stiffness is an objective of the structure then carbon is almost always used. This is why it is added to glass structures. It can be added to just certain areas (spars, fuse sides) to create the required stiffness. Having it everywhere isn't always necessary or beneficial.
Carbon fiber also blocks radio signals while Kevlar and fiberglass do not. In the modern age were 2.4 receivers have short antennas getting them outside the carbon fiber in varying orientations becomes difficult.
Carbon structures can be lighter for the same strength, but due to material availability it's not always a reality.

Weight: Carbon fiber is lighter than fiberglass, it’s 70% lighter than steel, 40% lighter than Aluminum.

Strength vs Rigidity: Carbon fiber and fiberglass are both very strong, but Carbon fiber is more rigid. Carbon fiber is about 3 times stiffer than steel and aluminum for a given weight.

In applications where a small amount of flexibility is allowed, we should use carbon fiber. Fiberglass is better suited to extreme flex patterns, while carbon fiber has a relatively small flex window.

Toughness: The shape of Carbon fiber doesn’t deteriorating slowly over time. That means the shape of carbon fiber doesn’t change when consistent and constant force is applied.
Fiberglass however being more flexible, it means that glass-fiber typically has a higher ultimate breaking point than a similarly shaped carbon fiber product, therefore having higher strength-to-weight ratio. In general it is a “tougher” material but the incredible rigidity of carbon makes it less capable of enduring certain abuses than fiberglass.

Characteristics: As opposed to most other materials, carbon fiber has a negative coefficient of thermal expansion which means they don’t expand or shrink as much as fiberglass when the temperature changes. This is a desirable quality for applications that have to operate in a wide range of temperatures.

Fiberglass composites are insulators, which means they do not respond to an electric field and resist the flow of electric charge. Fiberglass composites are very radiolucent, which means they allow radiation to pass through it freely. This makes fiberglass rods a useful material for antennas.

Price: Fiberglass is much cost effective. Long strands of carbon fibers are very difficult and expensive to manufacture, while fiberglass processes much easier. As a result, fiberglass is considerably less expensive than carbon fiber. Fiberglass composites are less expensive than carbon composites in most cases.

If stiffness isn't a driving factor then fiberglass is often very suitable. Fiberglass is easier, faster, and cheaper to use. Kevlar is very timely to cut compared to carbon and glass.

When it comes to toughness, glass can perform better than carbon due to its higher strain properties and be easier to repair.

• Foams:

EPO (Elapor is Multiplex's trademark foam) is a very durable foam, easily glued back together with medium CA (Cyanoacrylate, same as Loctite Super Glue 3) and kicker (accelerator, triggers quick polymerization of glue).

EPP is Expanded Polypropylene, and is highly durable, and flexes a lot, so generally, people use carbon flat spars or tubes with it to stiffen it. It is also glued back together with medium CA and kicker. Planes made of it generally bounce off of anything they hit.

EPS is Expanded Polystyrene if I remember right. It's basically Styrofoam/Styropor, which is commonly used in cheaper planes, and jets because it can be molded.

Depron is basically flooring insulation that was banned in the US for sale. It's a very popular material that is mostly used with 3D aircraft, because it yields very light wing loadings which allow the planes to basically float around. It's also used for sport jets, or unique aircraft because it's pretty easy to build with.

EPO and EPP are more durable than EPS and Depron.

Some examples:

Elapor

MultiPlex Easy Star

MultiPlex Easy Glider

Magister, MiniMag, Space Scooter, Twin Star II, just about everything from Multiplex

This type of foam is characterized by rather large 'cells'. It also has a slightly 'greasy' feel to it.

EPS Expanded PolyStyrene

GWS E-Starter

GWS Formosa I

GWS Pico Tiger Moth

GWS Tiger Moth 400

GWS DeHavilland Beaver

GWS PT-17 Stearman

Fine-beaded cells; EPS foam is very prone to hanger rash but is supposedly quite good at absorbing bad landings. More major crashes usually require collecting all the pieces in a plastic bag, but it's usually possible to just reassemble the jigsaw of pieces with glue. Sometimes referred to as 'beer cooler' foam.

Regular Styrofoam

Mountain Models MagPie

Mountain Models MagPie AP (Aerial Photography)

This is the unexpanded PolyStyrene. Very crumbly, with no defined 'cell' structure. This type of styrofoam must be protected from literally crumbling into thousands of tiny chunks of styrofoam by wrapping with tape. Once wrapped it becomes a very strong, durable and light material.

EPO - Expanded PolyOlefin

GWS AT-6 Texan

GWS F-15

GWS FW-190

GWS P-40

This is 'supposedly' identical to the Elapor foam of which the MultiPlex Easy Star is made. GWS is starting to make their newest planes of this material. Large beaded cells, and with the same 'greasy' feel to it.

EPP - Expanded PolyPropylene

AeroHog AeroAce

EPP foam doesn't dent or break. It has a 'spongy' quality to it, so on impact it compresses and pops back to shape.

Z-Foam

HobbyZone Super Cub

ParkZone F-27C

ParkZone T-28 Trojan

Similar in appearance and large cell size to Elapor and EPO, but without the 'greasy' feel to it. It's not quite as elastic as Elapor and EPO; in other words, a bit more brittle.

References:

http://www.rcgroups.com/forums/showthread.php?t=1631821
http://blog.oscarliang.net/carbon-vs-fibreglass/
http://www.rcgroups.com/forums/showthread.php?t=986842

 Conclusion

With all the collected data, the carbon fiber choice seems the most plausible option because of its strength and lightweight, at least for the plane’s main structure (spars, some ribs in the wings and fuselage and a lengthwise beam). Otherwise it increases too much the final weight, almost doubling it if we compare volumes of similar planes and change the density from foam to carbon fiber.

The shape of the fuselage can be made out of foam, with carbon fiber or fiberglass reinforcements in the areas with most risk of impact/crash.