3 Answers

Size.

A spinet piano has what is called a 'drop action'- A console piano has a 'forward blow-or direct blow action'. also the size of the piano measured vertically, a spinet piano is between 36" tall to no more than "42" tall, where a console size is 43" ~ 44". After this they are called 'studio uprights and full uprights.

In the years past, the spinet was very popular due to its small size, ironically, they are deeper than consoles, primarily because the 'action' (piano mechanisism) is mostly located at the bottm of the case. (drop action)

Spinet pianos are not popular anymore, as a matter of fact, there isn't a paiano manufacturer today that even makes them. They are not appreciated by teachers or musicians due to their weak 'tinny' sound and bad actions (feel)

The preferred is the console over the spinet 100% of the time.  Consoles and spinets are also designer cabnets that many people will buy to match their furniture, neither console or spinet are to be compared with uprights or grands.  There are some superior consoles out there, Yamaha is one, and if cost is not an object, the very best is Charles Walter, made in Elkhart Indiana but they are very pricy!

Source:  I do this for a living, I am a professional musician and I sell pianos for almost 30 years.. 

A Spinet rest on legs and a Console is enclosed all the way to the floor.  I think thats the differance

Someone tell me if I am wrong.