There are several things to consider, the most important of them being, Is plywood as strong as solid wood, and is spending money on solid wood furniture really worth the cost?
We will come to that, but first of all, here's a quick introduction of the two materials, so as to get on a common footing.
Plywood is an engineered wood product. It's made by glueing individual layers of plies (called veneers) over one another. These individual veneers are in turn obtained from logs by rotating them and peeling off thin layers from the logs.
Solid wood on the other hand, does not need much introduction. It's the natural wood that is obtained from trees. The trees are first felled, and the timber logs are then cut in the saw mills to smaller sizes and these wooden blocks are then used for making furniture. Amongst the most popular solid woods are teak wood, oak wood, cherry, maple, mahogany, and indian rosewood (sheesham wood).
Strength comparison between plywood and solid wood:
Solid wood is considered to be stronger compared to plywood since it is a homogeneous material. It also depends on which wood we are talking about. Good quality hardwoods (obtained from deciduous trees such as teak or sheesham) are denser, heavier and stronger compared to softwoods such as pine wood or mango wood.
Plywood on the other hand is made up of sheets that have been artificially glued together and if the 'Glue shear strength' is low, then the individual plies can come apart. This depends on the quality of the plywood though. The better quality plywood sheets in India are often made from hardwood veneers such as Gurjan wood, which is very stong.
Which one looks better?
For this point also, Solid wood again scores better than plywood. For example teak wood is so popular not only because its strong and durable, but also it looks nice with its straight wood grain patterns and its golden brown colour.
The faces of the usual commercial plywood available in local plywood shops is not decorative. So it requires additional cost and efforts to increase the beauty of plywood by glueing either natural wood veneers or decorative laminates (such as Sunmica or others) on its top faces.
Which of the two is easier to care for?
Laminated plywood surfaces are easier to maintain, because they are scratch resistant, moisture resistant and can just be wiped clean with a damp cloth. For best results, its better to go for a laminate that has a higher thickness. Very thin laminates can get damaged and are difficult to replace.
Good quality solid wood such as teak wood is water resistant, and great for outdoor furniture also. It does not require much care, though every once in a while (couple of years) it will become darker with age, and hence sanding and application of wood polish is required to help regain its original shine and colour.
Which one will last longer?
Depends on the wood used in both cases. Plywood of qood quality (Waterproof grade) is better than the commercial MR grade (moisture resistant) plywood. Marine grade plywood is even better than the two of them. Better the plywood is, the longer its life will be.
The durability of solid wood depends on the wood used. Hardwood (such as teak wood or sheesham wood) is stronger and more durable than softwoods such as Mango wood or Pine wood.
What about the cost?
Good quality solid wood is much costlier compared to plywood.
This is mainly because of the high demand and low supply. The supply of wood from forest reserves has to be controlled and kept limited because of ecological concerns, whereas the wood from plantations takes a long time to mature.
In my opinion, more than anything else, this low supply of natural wood has been the no.1 reason why newer engineered wood products such as particle boards (made from sawdust) and MDF (made from wood fibres) were able to gain a foothold in the furniture market.
Comparison based on the type of furniture:
For furniture such as book Shelves, and wardrobe doors, plywood is not the most suitable material since it has a tendency to bend in the middle when very long pieces are used. Better suited for such cases is blockboard (which is cost-effective) or solid wood.
For cabinets, solid wood is considered the best available choice, the next best can be veneered plywood or laminated plywood. For example, a solid teak wood cabinet would be better than teak veneered plywood, though it will also cost more. (A decorative veneered plywood is one, where the base material is plywood on top of which a better looking natural wood veneer has been glued).
For single and double beds, diwans and settess, the material used most often is plywood.
For doors, plywood is not the best choice, because of its tendency to bend. Considered better are solid core flush doors decked up with a decorative veneer, or solid wood doors.
For desks, chairs and dining tables, either solid wood or plywood can work equally well.
To summarize, I would say that natural solid wood is better than plywood in most respects. The only major downside is that it costs little more.