An ARF is (in some cases) an option at retirement allowing one to roll some or all of the maturing pension fund into a further investment fund rather than taking the benefits there and then. A PRSA is a pension option for saving towards retirement. An ARF may be an option for a PRSA or other pension holder at retirement age. They are not really the same thing and the idea of choosing an ARF over a PRSA doesn't really make sense as far as I know. Check the Pensions Board website for more on such terminology and options at retirement age.
Sorry guys, I meant to ask the difference between an AVC and a PRSA and why might one choose an AVC over a PRSA? Sorry, my brain is fried at this time of the evening!