NALC, as you know, is the National Association of Local Councils, membership of which is via a county office. They support local councils by providing lobbying, advice notes, a national conference, training opportunities for both clerks and councillors and a host of other things. Membership fees include a national fee element and local fee element and I agree are really expensive. I my opinion, the support is only as good as the local county office/officer although if, for example, you intend applying for a public works loan, this has to be done via your local county office. My own council have chosen not to be members of the county association after poor service but this does limit access to the national association so I'm not entirely supportive of that decision.
SLCC is the Society of Local Council Clerks and as the name suggests is an organisation which is supportive of Clerks in particular. They also provide training and in particular offer the training leading to the Certificate in Local Council Administration which your clerk must have if you intend applying for or operating under General Power of Competence. They also have an advice service, issue advice notes, run a conference, providing in particular a service updating clerks when there are changes in regulations/legislation, have a forum for clerks, continuing professional development requirement and many clerks (most?) are members and will use the advice and guidance to ensure they provide a professional level of service to their council.
NALC and SLCC work together on a range of issues such as agreeing industry standard employment contracts, pay scales etc.
It is a matter for individual councils to decide whether to pay the SLCC fee for their clerks but without a doubt councils do benefit from having an up to date qualified clerk. Those who work for more than one council usually ask their councils to contribute. There is a separate union for local council clerks (called the Association of Local Council Clerks) and the membership fee for that should be borne by the clerk themselves.