for a desktop the 15.6 will seem small. it is about the average laptop screen size.
I would not always go with bigger is better since using a larger TV as a monitor leads to much larger pixels(especially at 1366 x 768), something that is not an issue for a tv because it is made to be viewed farther away.
Another example would be how Nintendo made the 3ds XL with 90% larger screens and you can much more easily see the pixels.
Image quality is also another factor, but this varies from user to user.
At those 2 sizes the 18.5 is the better bet for sure. Wide screen unlike old 4:3 screens are actually quite small at 15.6 inches.