I belonged to the Guild once and still am a member of BASCA and have been for many years. The Guild in Cornwall once had Charlie Landsborough as a member. He was later signed by the same publisher as I was, and then in Ireland and his albums have sold well, and he is famous. A very nice person he was, too, when I had the pleasure of meeting him. The Guild once profiled members, including myself, with good articles which were interesting for other members and inspirational because they were about aspiring writers and lyricists. This was all some time ago, and so can't speak for what the Guild does now.
BASCA has changed a lot from when I first joined, both in the way of staff, which involves a lot more people now, and how the magazine is written. It is more about well-known writers now, and that must be how people want it to be. When I first joined, I could go to the small offices in London and have a chat with the General Secretary and assistant. They made you feel they knew you and would be as helpful as they could. With time, BASCA has moved on to bigger and better premises and so things are not on such a personal basis, where knowing each other is concerned.
New members would not have the past to compare with, and can take advantage of what is offered now to members, especially if they can get to London easily.
I have belonged to the ISA in Ireland for many years, and is great for prompt advice in a polite way, friendliness, and a good, helpful newsletter sent by email. I hope that this information is helpful and interesting.