The short answer: engineered. While there may be isolated times when a solid wood floor is the better choice, in 90+% of situations you are going to want an engineered floor.
So – what is the difference? As the name would suggest, a solid wood floor is composed of planks cut directly from a tree. There is a verity of ways to cut planks from a tree, the common being ‘plain sawn,’ ‘quater+rift,’ and ‘true quartersawn.’ The character, look, performance and practical uses of the wood depends on the wood species and how the wood is lumbered.
Engineered planks, on the other hand, are composed of a ‘finish’ or ‘wear’ layer laminated on a structural layer. The wear layer is formed of the wood that you will see – the finish. The structural layer is usually composed of plywood. The finer the product, the thicker the wear layer will be and the finer grade the plywood.
The biggest advantage of engineered flooring is it is stronger, more stable than solid wood flooring. The nature of the plywood structural layer is that resists bending, cupping, and curling significantly better than solid woods.
There are many myths and misconceptions about wood flooring. One being that solid wood flooring is more expensive and therefore better. For many common kinds of wood, solid wood flooring actually might be slightly less expensive, since the process of laminating the finish layer on the plywood and the plywood itself may be more expensive than the wood. For more precious woods, engineered it often less expensive because it uses less of the precious material. In short – there are expensive and inexpensive options in both engineered and solid woods. Typically, you can get engineered wood for less than solid wood, and the low end engineered wood can be very inexpensive.
Another myth is that an engineered floor can not be sanded and refinished or can not be refinished as many times as a solid wood floor. For most qualities engineered floors, you can refinish it as many times as a solid wood floor. Most floors are installed using a tongue and groove. This means, even if the floor is solid wood, you are not able to sand past the top layer above the groove. Engineered floors come in many kinds and qualities. A lot of this comes down to the thickness of the wear layer. High quality engineered floors will have a wear layer equal to that of a solid wood floor, usually +/- 7mm. Rule of thumb is that you get 1 sanding per millimeter, and floors need to be refinished about once every 7 to 10 years. So if you do that math – a quality engineered floor should last 50-70 years. Inexpensive options come with thinner wear layers that provide good options for people working on a tighter budget.
In closing – there are MANY options when it comes to flooring. Do not be turned off by engineered floors. They typically outperform their solid wood counterparts at every price point.