the functional difference is durabililty and adjustability.

the difference isn't really noticeable at first, but after thousands of miles and hundreds of adjustments the small parts holding the saddle clamp together usually wear out on the cheap posts (i'm sure there are exceptions).

posts designated as "micro adjust" (not just thompsons) allow you to change your saddle angle in smaller increments, which can be especially useful with brooks saddles as they sometimes have a pretty small sweet spot between pressing on your junk and sliding you off the front.

is all that worth paying 3-4 times the cost of a regular post? depends. i've never bought a thompson at retail price, and probably wouldn't unless my financial or riding situation changed. but if it did i'd certainly consider it.