It is difficult to compare solid wood flooring to engineered wood flooring due to the wide range of quality in both product categories, particularly engineered. Solid wood has some limitations. Recommended maximum widths and lengths are typically 5" / 127mm wide and 7' / 2100mm long. Solid hardwood is also more prone to "gapping" (excessive space between planks), "crowning" (convex curving upwards when humidity increases) and "cupping" (a concave or "dished" appearance of the plank, with the height of the plank along its longer edges being higher than the centre) with increased plank size. Solid wood cannot be used with underfloor radiant heating. However extra care is necessary with the planning and installation of the heating system and the wood flooring, such as limiting the temperature to 85 °F (29 °C), avoid sharp temperature fluctuations, utilizing an outdoor thermostat to anticipate heating demands, and monitoring the moisture content for the subfloor before installation. Hardwood Floor
Width & Length – Plank flooring comes in a variety of widths and lengths. The most common widths are currently in the 5” – 6” range, but wider widths are rapidly becoming more popular. The length of your floor is also important to consider depending on the size of your space. Hardwood flooring comes in mixed lengths because imperfections are cut out of longer planks during production. Rigid Core flooring is built to size and comes in lengths ranging from 36” up to 80+”.
Engineered wood flooring was created to lower the cost of wood flooring and attempt to fix some of solid wood’s shortcomings. It’s constructed from inexpensive plywood or particle board and topped with a thin veneer of hardwood. This construction helps with installation flexibility and slightly improves indent performance, but it adds a new set of problems as well. Because the “real” wood part of the floor is just a thin veneer, damage from scratches can be irreparable. Engineered wood floors are still made from porous wood, meaning they can’t be subjected to water.
You cannot restore every wood floor by simply refinishing it. If your wood floor has become warped or if several of the planks have become damaged, you will typically need to replace the entire floor, as finishing will only brighten an already-damaged surface. On really old floors, you may see the tongue where the boards come together, or your boards may lack adequate thickness. Floors like this are definite candidates for replacement, as refinishing is purely cosmetic. Denver Hardwood Floor
There are some characteristics that are common to each category: solid wood is more frequently site-finished, is always in a plank format, is generally thicker than engineered wood, and is usually installed by nailing. Engineered wood is more frequently pre-finished, has bevelled edges, is very rarely site-finished, and is installed with glue or as a floating installation. 
There are some characteristics that are common to each category: solid wood is more frequently site-finished, is always in a plank format, is generally thicker than engineered wood, and is usually installed by nailing. Engineered wood is more frequently pre-finished, has bevelled edges, is very rarely site-finished, and is installed with glue or as a floating installation.  Hardwood Floor
Solid hardwood floor planks are made from a piece of wood and made to be anywhere from 18 to 20 mm thick. The boards are made with a tongue and groove to fit together for installation. Depending on the species of wood, certain floors will be harder than others. The harder the floor, the less susceptible it is to dings and scratches. The floor can be sanded and refinished several times to restore its beauty, with the total number of times being determined by the depth of the tongue from the top of the plank. Denver Hardwood Floor Install
If you get a second opinion and they can’t be refinished, then your best option will be replace them…or wait until you can replace them. Given when the house was built I’m doubting the floors are even, so putting a floating on top will be a really bad option as they will wobble. And, installing wood on top of thin wood is a really bad solution. If your foundation is weak, your floor will be weak and it will creak and separate. Denver Hardwood Floor Install
Solid wood flooring is milled from a single piece of timber that is kiln or air dried before sawing. Depending on the desired look of the floor, the timber can be cut in three ways: flat-sawn, quarter-sawn, and rift-sawn. The timber is cut to the desired dimensions and either packed unfinished for a site-finished installation or finished at the factory. The moisture content at time of manufacturing is carefully controlled to ensure the product does not warp during transport and storage. Hardwood Floor