They are very different. "Showers" has high winds causing the rain to go nearly horizontal while "Rain" has no wind so it falls vertically.
Just be glad that Fair/Sunny and Clear are now counted as the same weather for the vistas and that the Skywatch NPCs make Dusk (5PM to 6PM) vistas far more sane. Those who did all of them before the Skywatch NPCs can tell you how sanity destroying some of the second set Dusk vistas were.
