Jim, you're right in that the terms have considerable overlap, even here in Oz but, depending on your background (both regional and temporal), they do invoke different things to many.

Leaving aside the requirements of scansion in the song, a small covered area just outside the entrance to the house might be called either porch or verandah but one that was large enough to have a group seated in it would usually attract the term verandah. That's the use of the two terms where and when I grew up; the back porch of our house in Melbourne was about 1m x 1m, whereas the front verandah was about 2m x 3m.

In O, we probably had more contact (via colonial connections) with people who had some connection with Burma, and so had more exposure to the term "verandah", a Burmese word that became applied to such structures.

Cheers, Rowan