I've been sitting on the sidelines watching this thread to see what might surface and felt that it was time to add my 2-cents worth.
The best job I ever had was as a professional model builder (20 years ago) for a consulting engineering firm http://www.rwdi.com/
. My job was to build scale models of bridges and buildings for use in wind tunnel and snow flume analysis prior to the expensive construction of full scale (real world) ones.
Most of the study buildings were made of sheet acrylic or polycarbonate with some small scale details made from sheet styrene.
The bridges where made from various combinations of balsa, thin aviation grade plywood, acrylic, polycarbonate, styrene, brass and aluminum; with the choice of material based on the type of the part, its load bearing requirement and the level of scale detail required.
Truss work was usually balsa . . . sometimes laser cut in a truss-like pattern, whereas roadway surfaces might be thin (1/32 - 1/16” thick) plywood. Piers might be acrylic, polycarbonate of in some cases machined aluminum. Suspension cables were made from monofilament line, very small gauge wire or in one case human hair . . . yes human hair because of its tensile strength vs diameter. We also made extensive use of CA glues and epoxy for bonding.
Pracas, your choice of material should be based on a number of factors:
*your desired aesthetic (a bridge made from popsicle sticks looks a lot different than one made of toothpicks or balsa)
*your materials budget and materials availability
*your capabilities as a model builder
If you do not have much experience working with tools and these types of materials, then choose a simple project made from simple inexpensive materials so that you do not get frustrated and give up midway through.