It may also depend on your eyes (over 40 - wear glasses?). Perversely, I've found the 27-inch more difficult to use than the 21.5-inch, because:
1. The resolution is higher (I think) on the 27-inch which makes text and all the menus etc slightly smaller to read, even though there is more screen real estate.
2. And the amount of LED light coming from the 27-inch - due to the surface area - is increased, which I also found more difficult to look at.
So I can view things better and more clearly on the the 21.5-inch than the 27-inch.
(N.B. this is highly subjective and other people will have different experiences).
Plus, going from a laptop the 21.5-inch is huge anyway.