Being an FCCA myself I would say ACCA of course.
ACA is becoming less popular, with graduates turning to ACCA in drove along with the Scottish Institute.
Traditionally ACA had more members as it was founded long before ACCA, but ACCA is fast catching up and is more global with membership and exams in many countries as far away as Asia.
ACAs tended to get paid more as it was deemed better but I think ACCA is catching upand at the rate it is growing, and the fact that Financial Standards are becoming Global Standards the ACCA may well overtake the ACA.