It may be local culture, but I had the impression that the general "Assistant Ward Clerk" was being avoided in favor of the specific Finance and Membership roles?
Any idea where this instruction (impression) is coming from? The Handbook is clearly agnostic about a preference to the type of Assistant Ward Clerk (general, financial, membership). My thought would be to think not from a calling perspective, but from a load perspective. If the Ward Clerk needs general support, Assistant Ward Clerk allows him delegate tasks as needed. But if the needs are specific to finance or membership, then that would seem proper. The Bishop will also surely balance the needs of administration, with other potential callings a worthy brother could hold in the ward. Each ward's circumstance will be unique.